53 overdrive recomended oil

The old manuals only state not to use hypoid lube which is GL-5. They state to use mild EP lube which was GL-3. Even though GL-3 is an obsolete rating I think there are still a few companies that blend it but you have to special order it. The GL-4 lubes have more extra pressure additive than the old GL-3 did but how much is too much, i don't know. Personally, I think they would be OK in there and NAPA sells Sta-Lube brand GL-4. If you want to use something that may be easier to get and more like GL-3, you can look for straight grade SAE 40 motor oil in cooler climates and SAE 50 in warmer climates. These motor oils are the same viscosity as SAE 80 and 90 gear lubes respectively since they aren't graded the same way. A lot of the old timers recommend the heavy motor oils. One reason is the yellow metals and the other is the sprag clutch for not using high EP lubricants.

I use Aeroshell 100W in mine since I also use it in the helicopter engines. This oil is a grade 100 SAE-50 aircraft motor oil with only ashless dispersant as an additive. Straight mineral Aeroshell 100 can also be used as it would be a GL-1 type but Aeroshell 100W plus (+) can not because the additive will cause the sprag clutch to slip so you might be better off just finding an automotive grade motor oil. They are around 80 grade and would work just fine in an overdrive.
 
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Those little R10 planet gears don't like SAE 140. It would work OK in the transmission side but some with leak over to the overdrive end.
 
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