The Stovebolt Site Forums The Shop Area Driveline '46 Rear axle and transmission oils

I'm sure this was covered before but I'm having trouble finding a reference.

Looking for recommendations regarding gear oil for my '46 3600's trans and rear axle. The original spec is SAE 90 Hyphoid gear oil but I need to translate that to product that is available today. Assuming that some modern oils might be harmful to the yellow metals. Suggestions or pointers to additional resources are welcome.

here is what Novak's says about the SM420 transmission, I think yours is a different tranny but, the the info has some good insights.

When filling your 420 with gear oil, we recommend that you select a conventional mineral oil or a para-synthetic in lieu of a full synthetic oil. Properly assembled manual gearboxes do not have the thermal strains seen by combustion engines or hypoid gears. Synthetic fluid in these gearboxes, while not harmful, is probably an economic waste.

Hypoid gear oil is sulphurized higher than transmission oil and can be mildly corrosive to the non-ferrous alloys used for synchros, bushings and thrust washers in these transmissions.

An 80W-90, API-GL5 or MT-1 rated fluid is very good. Some claim faster shifts from using a 50W engine oil in their transmission and we do not consider this to be contraindicated unless you operate your vehicle in a very warm environment.

I use a straight 90 weight GL-1 in the transmission as there are some bronze bushings on the counter shaft. GL-4 or 5 80W-90 for the rear axle.

Thanks, I found that NAPA has a SAE 90 GL-1 mineral gear oil that I will use in the tranny. For the differential, I plan to use Sta-lube SAE 85W90 Multi-purpose Hyphoid gear oil, API/GL-4.