80W-90 gear oil (for transmission and final drive)

[Mwether]

Oil Thread Warning! Mea maxima culpa!

I DID use the search function and read everything I could find. (Lots of it is dated, when it comes to the availability of certain products.) I may be more confused now than when I started! 

Still wondering about the following:

1. Dino gear oil is better than synth b/c it's less likely to leak? Also synth "eats" plastic transmission internals? Is that a real concern?

2. What about gear oils rated for limited slip diffs? (The current Castrol product is one.)
https://www.oreillyauto.com/detail/c/castrol/oil--chemicals---fluids/grease---lube/gear-oil---additives/80w-90/bfd65d65f2da/castrol-conventional-gear-oil-80w-90-1-quart/cas0/8090?

3. Does the GL-4/GL-5 distinction matter, or will either be okay?

4. Should I try to find an "extreme pressure" gear oil?

I ask the last question because the OEM oil was Agip Rotra MP. Agip merged into Eni. Eni now makes Eni Rotra MP, about which they say...

"Eni Rotra MP 80W-90 is an E.P. (Extreme Pressure) gear oil recommended for hypoid gears operating under severe conditions, and for very highly loaded gears in general, especially those subjected to dynamic loads. It is especially suited for axle drives with hypoid gears, which show very high loads, sliding contact and high temperatures. It can also be used in the gearboxes on cars and trucks, and in gearboxes, axles, final drives and power take-offs on farm and earth-moving machinery, and in general for all gears trains for which the manufacturer recommends an API GL-5 performance."

Maybe the essential question is: Which 80W-90 gear oil do you guys recommend in 2025?

[guzzisteve]

Just go to auto parts & get a qt/lt of 80/90 Valvoline & put it in no syn needed. Put black stuff in rear or not. ALL DONE, RIDE.

[smsw]

1.  Where did this come from?  If seals and gskets are in good shape, oil should not leak regardless of dino or synth.
2.  What does the mfr of your limited slip diff say?
3.  The difference is the amount of extreme pressure additives.  Depends on your application.
4.  Depends on the application.  Since an e.p. gear oil was OEM, I'd look for another brand w/ same specs.

[Tom H]

IMHO some answers.

?1) Allegedly Syn will clean the area that the seal is riding on. When clean, the old build up is gone that "may" have been helping the seal, seal. Now you have a leak. If you just built a 50year old motor and all new gaskets and seals installed, run either once broke in.

?1 again) Syn is supposedly bad for the plastic cage bearings per a bearing MFG (don't remember which MFG). I do not know if your bearings in the gear box have the plastic cages. Likely from the factory they did, but may have been changed?????

?3) GL-5 is fine for the rear and the gear box. GL-4 is safe for yellow metals like bushings in a 4 speed trans of a Loop.

?3 again) The manual calls for adding a Moly additive top the rear drive (NOT THE GEAR BOX). Supposedly the GL-5 gear oils do not require the Moly.

Just a suggestion. Get a quality brand gear oil that you like from the auto parts store. I had used Sta-Lube until it became hard to get and quite highly priced. A quality store brand should be fine, maybe check to see who it's made by. Lucas is supposedly a good gear oil???

Hope this helps,
Tom

[SemperVee]

 Not going to over think this.  ANY 85-190 or 75-140 will work for either I am seeing/reading and REDline is what I use right now. Changing is the important part and not seeing any metal particles. 

[Perazzimx14]

Any gear oil that meets or exceeds the manufacturers specifications in the right viscosity is what you should use. Brand name, price or fancy buzz words on the front of the bottle be damned. 

My personal favorite is SuperTech GL5 for the Hex/Camheads as it checks all the boxes the manufacturer requires and is cost friendly. Paying more does not always equate to getting more?

As a side not on dino oil being less likely to leak is bullocks. The oil base does not cause a leak a compromised seal not sealing (for any number of reasons) is the cause of the leak.

[Chuck in Indiana]

Yay, an oil thread.

As a side not on dino oil being less likely to leak is bullocks.

Sean Fader, a highly respected small block (and others) guy, told me to not run syn in the Lario rear drive. The outer seal area gets corroded,beat up, and will eventfully leak with syn. Maybe modern bikes are different? Dunno.

[faffi]

Yay, an oil thread.
Sean Fader, a highly respected small block (and others) guy, told me to not run syn in the Lario rear drive. The outer seal area gets corroded,beat up, and will eventfully leak with syn. Maybe modern bikes are different? Dunno.

I do not buy it. There is no reason why synthetic oil should cause leaks, but their better flow may expose a poor seal. Or some oils are less able to keep seals pliable, perhaps. My Kawasaki KZ400 had a small leak from the gear selector output shaft on dino oil that went away with synthetic from another brand. I do not think it had to do with mineral vs synthetic, but that the latter was of a better quality with better abilities to swell the seals in the engine. YMMV

[Perazzimx14]

Yay, an oil thread.
Sean Fader, a highly respected small block (and others) guy, told me to not run syn in the Lario rear drive. The outer seal area gets corroded,beat up, and will eventfully leak with syn. Maybe modern bikes are different? Dunno.

"The outer seal area gets corroded,beat up, and will eventfully leak with syn" 

[rocker59]

I've used Mobil-1 75w90 gear oil in transmission and bevel box for the past 30-years.

[Wayne Orwig]

SKF bearing warns you to not use poly caged bearings with synthetic, if they get hot. I have had to repair a LOT of old five speed gearboxes where the double row bearing have a poly cage blow up. I did not do a survey but most people had used synthetic. I always upgraded to steel caged bearings, no more issue.

[faffi]

SKF bearing warns you to not use poly caged bearings with synthetic, if they get hot. I have had to repair a LOT of old five speed gearboxes where the double row bearing have a poly cage blow up. I did not do a survey but most people had used synthetic. I always upgraded to steel caged bearings, no more issue.

I cannot find any warning like that on SKF's pages. The only info with some relevance is that if synthetic oil with Phosphate esters is used, max operating temp is 80C / 175F.

https://www.skf.com/group/products/rolling-bearings/principles-of-rolling-bearing-selection/bearing-selection-process/bearing-execution/cages

[Mr Revhead]

The stuff on synthetic is old wives tales made up by old bastards who refuse to accept new technology is better. Synthetic is better in every  single way.

Synthetic won't harm poly bearing cages. They will harm themselves! The issue is they are not best suited for the application. 

Redline heavy shock proof in gearbox and rear drive and be done with it

[Perazzimx14]

The stuff on synthetic is old wives tales made up by old bastards who refuse to accept new technology is better. Synthetic is better in every  single way.

Synthetic won't harm poly bearing cages. They will harm themselves! The issue is they are not best suited for the application. 

Redline heavy shock proof in gearbox and rear drive and be done with it

Redline Heavy Shockproof in this application is like killing flies with a hammer forge.

[Stretch]

Redline Heavy Shockproof in this application is like killing flies with a hammer forge.

They're dead, ain't they?!   

                                     -Stretch

[kballowe]

I ask the last question because the OEM oil was Agip Rotra MP. Agip merged into Eni. Eni now makes Eni Rotra MP, about which they say...

Mobil Delvac 1 is a virtual match to the AGIP/Eni Rotra MP.  It even carries the MT-1 rating (seal conditioner). 

Italy owns most of the petroleum concerns in country.  At one time they owned the majority interest in Moto Guzzi.  It makes sense that they would recommend a product from one of their companies.  Fast forward a few years, and now Moto Guzzi is on their own and they recommend "Castrol products".  Maybe Castrol gave them a better deal.

LOL - remember the heated discussions about only using AGIP/Eni branded products ?

Click on over to some of the shaft-driven motorcycle forums and hunt the threads on final drive lubrication.  Them boys use just about everything, with good results.

There's probably no wrong answer.  Everyone should use whatever makes them sleep better at night, assuming it meets the specifications as stated in the owner's manual.

[Moparnut72]

A dealer I knew back in the day always said he had seen more damage as a result of lack of than brand.
kk

[Perazzimx14]

They're dead, ain't they?!   

                                     -Stretch

But they are no more dead than if killed by a flyswatter.