Oil explanation for Guzzi (&BMW) owners: 75W90 is not an alternative for 85W90!

[Philnewbike]

You love a good oil thread, admit it    This is the mother of all oil explanations for why sticking to the manufacturer oil spec is usually the best bet. [edit: I have a 2005 Breva 1100... known as a CARC or big block]

In case you don't want to read on, this is the message: low W oils like 75W90 or even 80W90 (as replacement for 85W90) could be doing more harm than good in your gearbox... And if you never ride in sub zero temperatures or for lots of short journey then the W bit is largely immaterial anyway - see last paragraph from gearbox engineer

I've checked the temperature of my bike after stopping for about 2 mins after a long gentle ride (in the north of UK in June, sunny, ambient about 20, no wind - unusual!).
The temperatures in deg C were:

Crank ~85
Barrels ~95
Gearbox ~65
Shaft drive ~45

These make sense. When running the barrel temps were about 110 at the top. The gearbox is bolted onto the hot engine so as well as internal friction it receives heat soak and the final shaft drive is relatively insulated from the hot engine and is just getting hot from friction.

Lets look at viscosities at the operating temperature of the unit in question, in this case the engine (shaft drive and gearbox is further down the page).
Viscosity at 100°C engine operating temp:

10W60 Motul             23.5 mm²/s - Guzzi recommend 10W60
15W50 Motul             19.8 mm²/s
20W60 Millers ND      22.7 mm²/s
20W50 Millers           17.6 mm²/s
10W40 Miller             15.5 mm²/s
5W40 Motul              14.1 mm²/s
30 monograde Gulf    11.2 mm²/s
50 monograde Gulf    18.2 mm²/s

The 50 and 60 viscosities are within ~10% of one another at the operating temperature so are probably ok depending on whether you ride around Lake Como in summer or the Lake District in Spring. I listed monograde as this was probably avaialable when the V7 big block was designed. Interestingly (to me  ) the viscosity of 10W60 engine oil is not far off 90 weight gear oil until you go over 80 deg C, then it's significantly thicker (I plotted it). One more thing: when the original big block (i.e. the 1960s V7) was designed it would run rich (like all engines then) and therefore not quite as hot as engines now.

So lets look at the shaft drive and gearbox, for a 80W90 spec'd oil operating at about 45 degrees C in the shaft you need to know what the viscosity is at that temperature. Fortunately most oil manufacturerswill tell you the viscosity at two points: 40 and 100 C. I'll just assume 40 operating temp instead of 45 as it's easier:

Viscosity at 40 C shaft drive operating temp:
Agip 80W90         144 mm²/s - This is what Guzzi want you to use
Redline 75W90     100 mm²/s - THIS IS THIN
Miller ND 75W90    113  mm²/s
Agip 85W90      200 mm²/s  (Guzzi gearbox recommended... bit thick at 40 C)
Redline 75w140    175 mm²/s - looks fairly close to spec but you're paying for the protection of a a wide W oil and not actually using it! [/b]

Note that when a gearbox designer chooses an oil they do this based on what viscosity (and additives, e.g hypoid for shaft drive) are required at the expected operating temperature. I stress this because virtually every oil thread goes along the lines of "the manual says 90 weight so I use 75W90 super-duper $$$ oil because then get all the benefits of 90 and none of the disadvantages when cold". This is reasonable logic if you know nothing about gearbox design or oil but patently incorrect logic if you do as it will give you oil half as viscous at the actual operating temperature[/i]. The correct logic should be along the lines of: "at the expected operating temperature, my alternative oil has a similar viscosity, but is thinner when cold which helps for the first couple of miles". For the shaft that seems to be something like a 75W140. But note you're paying top dollar for a spec that is never used.

For the gearbox, which is what people are usually wanting to improve in terms of noise or feel, the story is slightly more tricky because if the expected operating temperature is ~65 deg C then we have to plot a line between the two temps the oil supplier gives us, i.e. from 40 to 100 C. I have done this because I am an obsessive compulsive!

Viscosity at 65 C gearbox operating temp:
Agip 85W90           74 mm²/s   - This is what Guzzi want you to use
Redline 75W90       47 mm²/s   - THIS IS THIN
Redline 75w140      81 mm²/s   - This seems similar but is expensive
Fuchs bus 90          70 mm²/s   - This seems similar but is dirt cheap
Fuchs BOA LS 90     82 mm²/s    - Similar, cheap and good quality
Eni Agip 80W90       54 mm²/s   - This is what Guzzi recommend for the shaft so if you put it in the gearbox as well it is too thin - is also recommended by several online oil selector tools


So, clear as mud now yeah ?? 


I'll leave you with this excellent snippet of information from this thread that lists an interview with Getrag (BMW gearbox builder) engineers, Klaus Sommer and Eberhard Schaetzle. SLightly different specs for BMW but I hope you can see the argument is similar, i.e. gearbox designers know what they're doing!  https://www.ukgser.com/forums/showthread.php/346792-R1100gs-clunky-gearbox

SCHAETZLE: Oil should be seen as an integral part of the transmission. When designing the transmission the load bearing capability of the oil is part of the calculation. We fill the BMW transmissions with SAE 90 GL 5 gear oil manufactured by Fuchs, a brand mainly found as an OEM supplier.
SOMMER: SAE 90-Oil should be used throughout the whole year. It istrue that in winter the shifting will suffer at first from the thick oil, but it should improve during a very short ride. For those to whom this is disturbing, because they make many short trips, for example, can use 75w90 GL 5 in winter as an alternative. In summer it must be changed back to SAE 90 GL 5.

[Kev m]

Great data - though you should probably note the bike you were using for standards and testing as the smallblocks and CARCs differ in certain recommendations and, I'm assuming may therefore differ on internal operating conditions.

Of course, I think smallblock recs vary too by model/year.

EDIT - ok, I pulled up some data that might help clarify.

CARC, 2TB smallblock Breva 750/Nevada,V7C/R, 1TB Smallblock (I, II, and III), V9, V85, Cali 1100 (hydro and flat tappet) and Cali 1400:

Engine Oil: all spec 10/60  (except hydro Cali 1100 which was 5/40, and flat tappet Cali 1100 could also use 20-50)

Transmission Oil (Gearbox in MG speak):

* 80/90 - 2TB smallblock (Breva/Nevada, and presumably V7C/R), and Cali 1100

* 85/90 (GL-5) -  CARC (B11 shop manual and 2006 Tech bulletin), V7 Mk I, and Cali 1400

* 75/90 (GL-5) - V7 III (and presumably II, I don't have a manual), V9

* 75/140 (GL-5) - V85

* 85/140 - CARC (B11 owner's manual) - presumably this is wrong, superseded by 2006 Tech Bulletin


Final Drive Oil (Transmission in MG speak):

* 80/90 (GL-4) - CARC (B11 workshop manual), Cali 1100 and Cali 1400

* 85/140  (GL-4 or GL-5) - CARC (B11 owner's manual but presumably wrong per tech bulletin), 2TB smallblock (Breva 750/Nevada/V7C/R [presumably]), 1TB smallblock (V7 I, II [presumably], and III, V9)

* 75/140 (GL-5) - V85



NOTE: I'm personally too removed from the CARCs now. I haven't had one in more than half a decade and I only have old tech data on the B11 so maybe someone could check/correct what I've posted here for that. I also found a discrepancy in the little information I actually have (Workshop vs Owner's manual contradict each other). Hell both of those publications are old enough that they actually have the engine oil a 5-40 and not the 10-60 to which the tech bulletin later updated the recommendation.

EDIT to EDIT - OK, I added the tech bulletin stuff too.

[Philnewbike]

Hi Kev. Thanks for the extra info, very interesting (to a madman like me), I have added my model now. Also, see my guesses at what the differences below mean. Cheers

Great data - though you should probably note the bike you were using for standards and testing as the smallblocks and CARCs differ in certain recommendations and, I'm assuming may therefore differ on internal operating conditions.

Of course, I think smallblock recs vary too by model/year.

EDIT - ok, I pulled up some data that might help clarify.

CARC, 2TB smallblock Breva 750/Nevada,V7C/R, 1TB Smallblock (I, II, and III), V9, V85, Cali 1100 (hydro and flat tappet) and Cali 1400:

Engine Oil: all spec 10/60  (except hydro Cali 1100 which was 5/40, and flat tappet Cali 1100 could also use 20-50) 10/60 is the same as my carc but guess they realised the hydraulic tappets only work well with thinner 5/40 oil at least in first few minutes. So they decided to sacrifice some protection for a quiet bike that customers don't take back to the workshop claiming it's noisy!

Transmission Oil (Gearbox in MG speak):

* 80/90 - 2TB smallblock (Breva/Nevada, and presumably V7C/R), and Cali 1100 I can see small blocks not needing quite as heavy oil but not sure why Cali 1100 wouldn't unless they're logic is it's not driven as hard, ie on freeway at 55mph lol...?

* 85/90 (GL-5) -  CARC (B11 shop manual and 2006 Tech bulletin), V7 Mk I, and Cali 1400 I guess for v7 mk1 they just played safe before deciding on 80W90 later??

* 75/90 (GL-5) - V7 III (and presumably II, I don't have a manual), V9  I think lower power, higher internal gear speed => thinner oil is ok

* 75/140 (GL-5) - V85   I would say that this style of bike might be doing anything from pottering to the shops to very high load / high temp off roading. To cover all possibilities without need to suggest multiple specs they just filled it with a modern very wide multigrade for simplicity (it's not as expensive as it once was either)

* 85/140 - CARC (B11 owner's manual) - presumably this is wrong, superseded by 2006 Tech Bulletin  Yep that seems to be the case


Final Drive Oil (Transmission in MG speak):

* 80/90 (GL-4) - CARC (B11 workshop manual), Cali 1100 and Cali 1400

* 85/140  (GL-4 or GL-5) - CARC (B11 owner's manual but presumably wrong per tech bulletin), 2TB smallblock (Breva 750/Nevada/V7C/R [presumably]), 1TB smallblock (V7 I, II [presumably], and III, V9)   This one's more of a puzzle: yes breva spec is wrong, but why would a small block need heavier oil here... are the gears a larger pitch (coarser) perhaps? Whatever it is I would assume they're running a bit hotter than the CARC bikes do.

* 75/140 (GL-5) - V85  See v85 point above: grade for very wide range of expected uses



NOTE: I'm personally too removed from the CARCs now. I haven't had one in more than half a decade and I only have old tech data on the B11 so maybe someone could check/correct what I've posted here for that. I also found a discrepancy in the little information I actually have (Workshop vs Owner's manual contradict each other). Hell both of those publications are old enough that they actually have the engine oil a 5-40 and not the 10-60 to which the tech bulletin later updated the recommendation.

EDIT to EDIT - OK, I added the tech bulletin stuff too.

[Kev m]

Hi Kev. Thanks for the extra info, very interesting (to a madman like me), I have added my model now. Also, see my guesses at what the differences below mean. Cheers

I'm guessing you're a bit like me and you enjoy getting lost in the minutia. 

Some thoughts on your SWAGs below. My new ones in red.


Engine Oil: all spec 10/60  (except hydro Cali 1100 which was 5/40, and flat tappet Cali 1100 could also use 20-50) 10/60 is the same as my carc but guess they realised the hydraulic tappets only work well with thinner 5/40 oil at least in first few minutes. So they decided to sacrifice some protection for a quiet bike that customers don't take back to the workshop claiming it's noisy!

If you're not familiar with the hydro debacle they had multiple recalls as the camshafts and lifters failed. I can't begin to guess at their logic, I just know the last recall seemed to do the trick, but by then the concept had such a black eye they gave up and went back to flat tappets (and eventually experience new failures with them a decade later on the 8Vs before going to roller rockers).



Transmission Oil (Gearbox in MG speak):

* 80/90 - 2TB smallblock (Breva/Nevada, and presumably V7C/R), and Cali 1100 I can see small blocks not needing quite as heavy oil but not sure why Cali 1100 wouldn't unless they're logic is it's not driven as hard, ie on freeway at 55mph lol...?

* 85/90 (GL-5) -  CARC (B11 shop manual and 2006 Tech bulletin), V7 Mk I, and Cali 1400 I guess for v7 mk1 they just played safe before deciding on 80W90 later??

* 75/90 (GL-5) - V7 III (and presumably II, I don't have a manual), V9  I think lower power, higher internal gear speed => thinner oil is ok

Actually with the smallblocks the progression is 80/90 on the older 5-speeds, 85/90 for the Mk I V7, and then the MK II and III changed to 6-speeds (and that's how the V9 debuted) with half the capacity and an oil pump and went with 75/90. But keep in mind that all the hemi-head smallblocks make more power than the previous generations of smallbocks.
 

Final Drive Oil (Transmission in MG speak):

* 80/90 (GL-4) - CARC (B11 workshop manual), Cali 1100 and Cali 1400

* 85/140  (GL-4 or GL-5) - CARC (B11 owner's manual but presumably wrong per tech bulletin), 2TB smallblock (Breva 750/Nevada/V7C/R [presumably]), 1TB smallblock (V7 I, II [presumably], and III, V9)   This one's more of a puzzle: yes breva spec is wrong, but why would a small block need heavier oil here... are the gears a larger pitch (coarser) perhaps? Whatever it is I would assume they're running a bit hotter than the CARC bikes do.

Yeah that's my sway - the rear drive on the smallblocks is that much smaller overall, but aside from lower hp output there are likely similar forces overall. I'm just guessing it's a harsher environment overall than on the larger/beefier CARCs.

 

[Philnewbike]

Hi Kev
Good discussion. I would guess that the addition of a oil pump meant that 75W was now a necessity on a (potentially) zero degree day when 80W or 85W might be too thick to pump affectively whereas an old model will simply drag oil up due to the dipped gear system of lubrication. Not sure about change from 80W90 to 85W90 unless that was just another a typo!
cheers
phil

[John Warner]

My CARC Oil (2009 Stelvio) has never got any hotter than around 40°C/104°F (I have a Temp Sensor in the Fill/Level Plug).
That was in ambient temps of around 24°C/75°F, and riding fairly 'briskly'.
I'll move the Sensor to the Gearbox at some point soon, just out of curiosity.

[Kev m]

My CARC Oil (2009 Stelvio) has never got any hotter than around 40°C/104°F (I have a Temp Sensor in the Fill/Level Plug).
That was in ambient temps of around 24°C/75°F, and riding fairly 'briskly'.
I'll move the Sensor to the Gearbox at some point soon, just out of curiosity.

How accurate is the sensor in your estimation.

And how much does that reflect the peak internal temperature that the oil can reach in the passageways of say the head or when squirted on the pistons?

I ask because I used to take infra-red head temps on a number of my Guzzis and Harleys. IIRC (and it's been a while) I would regularly see head temps in the high 200's (F) on my B11. I would think that oil in the exhaust valve cooling passage and hitting the piston would be getting hotter, even if that heat is shed by the time it gets back to the sump.

I've not had any sump temperature gauges on my Guzzis, but I've had them on a number of Harley Sportsters and peak head temps hitting mid 200's (F) might show up as remote oil sump temps in the 160-180F range while head temps in the low 300's might show as high as 210-230 (F) in the remote sump.

[Wayne Orwig]

Engine Oil: all spec 10/60  (except hydro Cali 1100 which was 5/40, and flat tappet Cali 1100 could also use 20-50) 10/60 is the same as my carc but guess they realised the hydraulic tappets only work well with thinner 5/40 oil at least in first few minutes. So they decided to sacrifice some protection for a quiet bike that customers don't take back to the workshop claiming it's noisy!

If you're not familiar with the hydro debacle they had multiple recalls as the camshafts and lifters failed. I can't begin to guess at their logic, I just know the last recall seemed to do the trick, but by then the concept had such a black eye they gave up and went back to flat tappets (and eventually experience new failures with them a decade later on the 8Vs before going to roller rockers).

I always like a good oil thread...... (not)
Just to toss in my $0.02 on the hydro thing. My take on the attempts to fix it where that some engineer thought that if a hydro lifter leaked down, the initial pounding as it pumped up, was the entire problem. Thus the 5w40 to get them to pump up fast when cold, and even an attempt at shimming the lifters, to reduce the amount of hammering on cold start. But even after they discovered that it was a valve spring issue, they stayed with the 5w40. I still use 5w40, but have a feeling that a 10w40 is probably a bit better for that motor. I have no proof, and I am dumb, so I stick with the 5w40.

Flame away, just thinking out loud.

[Philnewbike]

I really like your scientific bent chaps. Be curious to see what real oil temps you find Wayne, as opposed to my IR temps... assume similar if final drive was similar. As Kev rightly says there is still the point that oil actually gets much much hotter in the engine as it's used to some extent as a coolant. But the main point I wanted to make came about because several online oil retailers and big oil companies 'oil selectors' said 75W90 (or 80W90) for the gearbox and final drive and that is simply wrong. If anything, for most riders, straight 90 weight would be best for gearbox.
Cheers
phil

[Old Jock]

Shockproof heavy is popular with a lot of owners and many have used it in the box for many thousands of miles without issue.

From your data the Shockproof Heavy seems to be pretty close to what Guzzi quote viscosity wise at your running temperature, for the CARCs

I'm running it at the moment, in an older 5 speed box which quotes 80W-90 but as long as the oil is changed regularly and there's enough of it, I'm fine

I agree a wide multigrade is a bit pointless as the gear and bevel box don't run very hot in comparison to ambient, in the UK its definately not an issue.

As far as the engine is concerned, the older bikes quote a 20/50 I'm running a 15/50 full synth in an LM 1000, Sport 1100 and a HiCam engine. The difference in oil temp and pressures in these bikes is markedly different. The HiCam is the problem child and runs in clear air 20C ambient, around 110C, sump temp and overheats in traffic PDQ, pressure also starts to drop through the floor. When I say overheat sump temps 120-130C. I'm upping to a 10/60, as many who run HiCams do and overfill the sump by around 10-15 mm above the max level, but that's another story. IMHO factory specs are to be used where no data exists to prove otherwise, but shouldn't be regarded as set in stone, if you've got data that suggests a possibly better alternate

The QC at least on my old bikes were variable to say the least, and one bike out the factory might run hot another cold, or one may have good oil pressure another not.

Unless you gather the data to know what you're pressures and temperatures are, you're could be whistling in the wind, at least for the older models (up to early into the 2000s)

Guzzi used straight cut gears in some 5 speeders then went to helical. I'm no tribologist but I would have thought that these might require different oils, not according to Guzzi anyway. Perhaps I'm unduly hard on Guzzi but I do sometimes wonder how much engineering really went on at certain times.

Thanks for the information though it's nice to see somebody take a step by step logical approach to the issue and narrow down alternatives in a logical manner, appreciate the data 

[Two Checks]

What ever happened to good old yak fat?

[berniebee]

I've got a point to make, so bear with me while I go over some things. Oil varies in viscosity with temperature- the hotter the oil, the lower the viscosity. But it's important to note that this change is not linear with temperature.
The red curve below shows viscosity vs temperature for 10W60 engine oil, and gear oil has the same hyperbolic curve.
   



The second point I want to make is that the operating temperature of the engine, transmission and rear wheel drive varies directly with the daily ambient temperature. In other words, if your engine's operating temperature is 90C today at an ambient 20C, the engine temperature will be 95C when the temperature hits 25C tomorrow . Same story for the transmission and rear wheel drive.
 
Now, let's look at the right portion of the red curve. This part of the curve looks almost flat, doesn't it? This shows that the viscosity at about 100C doesn't vary much with a change in temperature.  So for an engine at operating temperature, a change in ambient temperature is not going to affect the viscosity much. (Yes, the oil will breakdown more quickly at higher temperatures, but that's another discussion.)

But it's a different story for the left side of the graph.  Viscosity varies wildly at cooler temps. Note how the graph shows a viscosity of 450 at 20C (room temperature.),  275 at 30C, 175 at 40C, about 100 at 50C, and 75 at 60C! These are all "operating temperatures " of the rear wheel drive, depending on the climate and season you are riding in. Even with the owners using the same brand of oil, the rear wheel drive of a Florida Guzzi consistently sees an oil that has a fraction of the viscosity of a rear wheel drive in rainy, cool England.
 
Conclusion: For the rear wheel drive, oil type is not that critical folks. These rear drives are designed to tolerate a huge variation in oil viscosity. Add moly* and you can use any of the oils mentioned in the OP's list.

* Or ensure your choice of oil has moly.

[moto-uno]

  The only part that's unfortunate here is that other than Motul ( short for bring cash ) is none of the other oils listed are
available here in B.C ( and probably the rest of Canada ). It'd be nice to see Pennzoil , Amsoil , Quakerstate or Castrol
checked for a future in depth check :) . Thanks for the read , Peter

[jcctx]

Gear oil good~ use some!!!

[ozarquebus]

What I do not understand is how Redline MT-90 fully synthetic gear oil can be substituted for motor oil, but I read that somewhere...

[Philnewbike]

As far as the engine is concerned, the older bikes quote a 20/50 I'm running a 15/50 full synth in an LM 1000, Sport 1100 and a HiCam engine. The difference in oil temp and pressures in these bikes is markedly different. The HiCam is the problem child and runs in clear air 20C ambient, around 110C, sump temp and overheats in traffic PDQ, pressure also starts to drop through the floor. When I say overheat sump temps 120-130C. I'm upping to a 10/60, as many who run HiCams do and overfill the sump by around 10-15 mm above the max level, but that's another story. IMHO factory specs are to be used where no data exists to prove otherwise, but shouldn't be regarded as set in stone, if you've got data that suggests a possibly better alternate

Thanks for the information though it's nice to see somebody take a step by step logical approach to the issue and narrow down alternatives in a logical manner, appreciate the data 

Thanks for that.
BTW you might want to consider an oil designed to run hot ?  https://www.opieoils.co.uk/p-1006-red-line-high-performance-synthetic-motor-oil-20w-50.aspx  (no data sheet though so no idea on viscosity)

[Stretch]

Interesting thread with input from some very knowledgeable people.

But we're pretty deep into the weeds here!

I can remember reading some spirited discussions about how the oil
in UJMs was SO inadequate for the transmissions, and how they'd
wear out in no time, yet there are plenty of unit construction bikes
tooling around quite happily with 100K-plus on the clock.

Use your favorite flavor of hypoid gear oil, and enjoy the ride. The
mechanicals will probably outlast you.

I'd bet you could get a surprising amount of miles out of a rear end
lubed with ATF. Some body ought to try that.......
 
                                                            -Stretch

[Philnewbike]

Hiya. My replies below in purple in good faith (Oh no: oil thread descends into typical oil thread thread... play nicely children lol.)

I've got a point to make, so bear with me while I go over some things. Oil varies in viscosity with temperature- the hotter the oil, the lower the viscosity. But it's important to note that this change is not linear with temperature.
The red curve below shows viscosity vs temperature for 10W60 engine oil, and gear oil has the same hyperbolic curve.

It isn't hyperbolic but exponential gives a reasonable fit. So what I did was plot a straight line between data points on a *log plot*, hence my interpolations are on an exponential plot- so no, they are not linear. See here for log-linear viscosity plot and how it is fairly straight (i.e. exponential) from 40 to 100 C  https://www.engineeringtoolbox.com/dynamic-viscosity-motor-oils-d_1759.html     

The second point I want to make is that the operating temperature of the engine, transmission and rear wheel drive varies directly with the daily ambient temperature. In other words, if your engine's operating temperature is 90C today at an ambient 20C, the engine temperature will be 95C when the temperature hits 25C tomorrow . Same story for the transmission and rear wheel drive.

Hmmmm, I need to do a bit of thermodynamics on that 😊 I think you’re right according to convective heat formula (heat energy proportional to change in temp of object and air flow) but I think it may be more complex than that… will have a think! But yes on first though it seems my 45 deg C shaft in ambient temp of 20 may run at 65 deg C in ambient temp of 40 deg C... not 100% sure tbh
 

Now, let's look at the right portion of the red curve. This part of the curve looks almost flat, doesn't it? This shows that the viscosity at about 100C doesn't vary much with a change in temperature.  So for an engine at operating temperature, a change in ambient temperature is not going to affect the viscosity much. (Yes, the oil will breakdown more quickly at higher temperatures, but that's another discussion.)

I've plotted *real* data as an exponential function - it does change viscosity. It looks like it doesn't because you're looking at a linear-linear plot and it's hard to see. As an example, from 90 to 100 C the viscosity of high quality Motul fully synth 10W60 changes from about 30 to 20 mm/s^2 (this will be more for mineral). That's not bad or wrong or something to worry about, it's just interesting... You think 10 mm/s^2 change in viscosity is a smaller value compared to the change at the cooler end. And you're right. But as a percentage change in viscosity it is quite large, i.e. decrease of 33% from 90 to 90100 deg C. If you sit at the lights on tickover for a long time on a hot day and your engine oil gets hotter and hotter with no air flow to take the heat away and the oil light comes on: that's why, lower and lower viscosity


But it's a different story for the left side of the graph.  Viscosity varies wildly at cooler temps. Note how the graph shows a viscosity of 450 at 20C (room temperature.),  275 at 30C, 175 at 40C, about 100 at 50C, and 75 at 60C! These are all "operating temperatures " of the rear wheel drive, depending on the climate and season you are riding in. Even with the owners using the same brand of oil, the rear wheel drive of a Florida Guzzi consistently sees an oil that has a fraction of the viscosity of a rear wheel drive in rainy, cool England.

I beg to differ (not done full calculations though so happy to be persuaded!). I reckon the oil operating temp after say 20 miles of riding will be similar whether ambient temp of 5 deg or 40 deg C (typical extremes of riding temps). Yes hotter in Florida, cooler in England. But I bet my house a bike ridden at 40 C and one ridden at 0 C will not have 40 C difference in shaft or engine temperature as your logic has it above - that's instinctively wrong but according to simple convection theory it is right... surely is more complicated than that no? BTW, the change only *looks* larger at lower temperature. Do the maths and you'll see it is just a trick of the exponential (approx.) curve - plot it as log-linear and it's much clearer!
 
Conclusion: For the rear wheel drive, oil type is not that critical folks. These rear drives are designed to tolerate a huge variation in oil viscosity. Add moly* and you can use any of the oils mentioned in the OP's list.

Remember folks, see the passage I posted from BMW Getrag engineers in the last paragraph of my post regarding their view of replacing 90 with 75W90... if you always ride in cold climate, yes can use 75W if you want, if you usually ride in mild climate just stick to the spec. And if you thrash it is hot climate maybe go more viscous...... i think

* Or ensure your choice of oil has moly.

Good to have the brain cells stretched, cheers 😊

[kballowe]

This five gallon bucket says "multi-purpose" on the label, so it must be good stuff.
It would be a lifetime supply.

[wymple]

I've spent my entire riding life with drive boxes filled with any weight I found on sale. Because I'm an old fart, I was always a fan of STP. It was the go to solution to keeping old worn out junk on the road back in the day. To this day I still put an ounce or two in my final drives. I have never lost a gearbox nor come close. I doubt I'm about to start worrying now.

[Philnewbike]

Yep maybe true, unless you're racing in Riyadh.
I think for gearbox protection and change quality it's probably important though.
Cheers
p

[SmithSwede]

Interesting thread.   And yes, I follow the Owner’s Manual and run the recommended 85-90 oil in the gear box of my V7 smallblock (which is a lot harder to find than 80-90).

Here’s my concern with this analysis.  The starting premise is that Guzzi makes recommendations in the manual which are the conclusions of deep technical analysis and thus the Baseline Platonic Truth.  What possible reason do we have to believe this is some profound truth, given we know that the spec can vary over time, that Guzzi has botched the spec with the hydro debacle, and given that oil companies pay off manufacturers to recommend their products?

What evidence do we have that if you ride normally and change the oil somewhat regularly, that these viscosity variations make a practical difference?

For most people, there is no practical difference between a gearbox that can last “only” 150,000 miles on the “wrong” oil but 300,000 miles on the “right” oil.   Why?  Because they junk the bike well before 20,000 miles. 

I suspect the biggest factor for longevity is probably regular oil changes to remove water contamination and metal shards.  And I bet it matters a lot that some people shift smoothly and accelerate smoothly and thus keep their gears happy, whereas other bang on the box and otherwise torture it. 

I’d certainly be interested in data from, say, a bike rental fleet that showed a real world difference just based on gear oil viscosity differences. 

Maybe the old moly recommendation is part of that data?  But my understanding is that the GL5 spec obviated the need for moly.   What would happen to gears that didn’t get the moly treatment?  And did any of that have anything to do with viscosity?

[Stretch]

What evidence do we have that if you ride normally and change the oil somewhat regularly, that these viscosity variations make a practical difference?

Exactly!

During WWII, they did all kinds of analysis on engines, metallurgy, etc..  Before the common use of oil filters,
they found that a magnetic drain plug made a HUGE difference to the longevity of internal engine components.

Change your oil and filter on a regular basis, your air filter on a regular basis, (or use a K&N - another K&N thread
coming on!  ), use magnetic drain plugs, and use whatever oil floats your boat that's close to original spec or better.

Then, RIDE AND HAVE FUN!   :-)

                                                         -Stretch

[Tom H]

Since were are on gear oil.

GL4 or 5. Did I read right that a rear drive is specifying GL4? I thought that went away with the Loop 4 speed gearbox?

EP (Extreme Pressure) additives? I was reading the manual for my '62 F100 about what oil to use in the gearbox, rear drive and steering box. I had read/heard somewhere that it was 20wt motor oil in the gearbox. I wanted to make sure what it needed. It said to use 90wt gear oil NO EP ADDITIVES or 50wt motor oil in the gearbox and I think rear drive as well. I looked at my jugs of 85/90 GL4 and GL5. GL4 has the EP, the GL5 did not say it on the label. Maybe GL5 has EP additives built into the formula, not an additive?

Why would Ford specify NO EP in a 4 speed granny 1st gearbox??

Tom

[RinkRat II]

     EP additives and Brass synchros don't play well with each other.

       Paul B

[Philnewbike]

Interesting thread.   And yes, I follow the Owner’s Manual and run the recommended 85-90 oil in the gear box of my V7 smallblock (which is a lot harder to find than 80-90).

Here’s my concern with this analysis.  The starting premise is that Guzzi makes recommendations in the manual which are the conclusions of deep technical analysis and thus the Baseline Platonic Truth.  What possible reason do we have to believe this is some profound truth, given we know that the spec can vary over time, that Guzzi has botched the spec with the hydro debacle, and given that oil companies pay off manufacturers to recommend their products?

What evidence do we have that if you ride normally and change the oil somewhat regularly, that these viscosity variations make a practical difference?

For most people, there is no practical difference between a gearbox that can last “only” 150,000 miles on the “wrong” oil but 300,000 miles on the “right” oil.   Why?  Because they junk the bike well before 20,000 miles. 

I suspect the biggest factor for longevity is probably regular oil changes to remove water contamination and metal shards.  And I bet it matters a lot that some people shift smoothly and accelerate smoothly and thus keep their gears happy, whereas other bang on the box and otherwise torture it. 

I’d certainly be interested in data from, say, a bike rental fleet that showed a real world difference just based on gear oil viscosity differences. 

Maybe the old moly recommendation is part of that data?  But my understanding is that the GL5 spec obviated the need for moly.   What would happen to gears that didn’t get the moly treatment?  And did any of that have anything to do with viscosity?

Yes I agree regarding MG. Basically I just do this for kicks as I'm interested in the science. Although I am an engineer I am not a gearbox designer or oil tribologist but I do work with some people who are. They use alll sorts of (largely empirical) charts to spec oils for gearboxes... there's plenty of research been done. As an example I was told that a too thick oil can actually give a smaller film protection because the high viscosity heats up the oil (in the contact point) and it momentarily becomes very hot and thinner than a lower viscosity oil would for that application... so 140 weight may not necessarily be better (I don't know, it would need a proper experiment to prove in this application... maybe it is).
I suppose the reason I've started this thread with it's pseudo scientific bent is because so many oil threads are along the lines of "I've always used gravy granules in my shaft drive and my bikes have never let me down, QED", or "I've always used 75W90 and my bike's never gone wrong, QED", or some such highly subjective opinion based usually (not always) on doing a few 1000 miles on a bike before it is passed onto the new owner. I just wanted to inject some engineering. But I am no expert but from what I've been told and read about I would suggest the process MG went through is simply asking Agip what they thought would be best based on: speed, load, expected normal ambient temperature range, gear teeth pitch, clearance and finish [edit: and cost].
I think the past paragraph of my original post from the Getrag engineers sums things up best when they explain why 75W90 is not an acceptable alternative for 90 weight in their box*
* most 85W90 oils are also classified as 90 weights

[Philnewbike]

Since were are on gear oil.

GL4 or 5. Did I read right that a rear drive is specifying GL4? I thought that went away with the Loop 4 speed gearbox?

EP (Extreme Pressure) additives? I was reading the manual for my '62 F100 about what oil to use in the gearbox, rear drive and steering box. I had read/heard somewhere that it was 20wt motor oil in the gearbox. I wanted to make sure what it needed. It said to use 90wt gear oil NO EP ADDITIVES or 50wt motor oil in the gearbox and I think rear drive as well. I looked at my jugs of 85/90 GL4 and GL5. GL4 has the EP, the GL5 did not say it on the label. Maybe GL5 has EP additives built into the formula, not an additive?

Why would Ford specify NO EP in a 4 speed granny 1st gearbox??

Tom

Some additives will attack yellow metals used n old boxes. Also, low friction modifiers will kill synchromesh gears because they rely on friction to work properly. So for that box and good protective film + not super slippery +no additives bad for the metals used will probably be best.

[Philnewbike]

What I do not understand is how Redline MT-90 fully synthetic gear oil can be substituted for motor oil, but I read that somewhere...

90 weight gear oil is actually a similar weight to 40/50 engine oil (different scale used) so yes, would probably work fine if you need to top up and had no 20w50... as long as that gear oil doesn't react with any yellow metals (is there in an engine??) in your engine and destroy it 

[moto-uno]

  I'd like to add , if any of these "gear oils" are thought to be usable in an engine , make sure they don't have sulfur as an extreme pressure agent .
Why not , well sulfur and water can be acidic and tend to attack shell bearings . How long ? Don't know and don't care to find out :) . Peter

[Tom H]

Thanks for the replies for my F100 gear oil. This makes sense. Loop 4 speed gear box has bushings that GL5 can hurt. IIRR specs say GL4. My F100 gear box probably has the same and synchros. BUT... the Sta Lube GL4 has EP additives per the label, maybe I shouldn't use that in a Loop 4 speed or my F100 gear box??? Maybe I need to find a NON-EP GL4 grade gear oil?

Maybe for the F100, use 50wt motor oil as a substitute like it says? Maybe even the 4 speed if the EP is bad for bushing?

Geez, this used to be simple. Grab a jug of 90wt and pour it in the gear box and rear drive of any vehicle you have. Done!

Thank you.
Tom