I had the first 1978 G5 in the USA
I was the second owner. First owner was Charlie. Pineiro who lived on Seagar Ave in the Bronx, near the Whitestone Bridge.
Charile knew Berliner the importer at the time personally and he bought it right out of tbe warehouse. I bought in in 1979 with 35000 mile on the odo.
If you have the original brochure of the G5 it would have a G5 with a person standing next to it standing under a sto e bridge wearing a light brown leather coat .
The story Charlie told me is he bought the bike so fast they didn't have time to take pictures for the brochure so he rode it out to that park somewhere north of the city (NYC) that was frequently with motorcyclists on the weekend and they took the pictures, however the person in the picture is not me just a point of Interest.
Now to get back to your question on gear oil.
At the time in 1979 I switched the entire bike over to synthetic oils , transmission and rear differential .
I was using Mobil 1 75/90 synthetic in trans and rear diff, the engine I was using 20-50 Klotz synthetic oil .
I sold the bike the odometer showed 165000 miles but it was more than that ,
I would say 200,000 plus would be a fair guess, did the transmission leak yes, my leak happened to be from the throwout bearing o-ring had collapsed and the shift Rod had to funnel shaped cones on the shaft to prevent oil leaking i to the clutch,that was later changed over to the O-ring version of the seal and never had any more leaks.
TOMB