"Why would the older transmissions use different oil? Bearing material? Just asking cause I never thought about it or figured there was any difference."
Joe,
My restored '36 Chevy PU has the same basic 3-speed transmission that was used in '32-'36 Chevy Master cars and pickups. The oil specified for those transmissions "back in the day" is SAE 160. Today stores like The Filling Station sell so called "600W" oil, claiming it is the equivalent of SAE 160 gear oil. Saying what follows here angered the owner of The Filling Station. Oh well. "600W" oil is not even a gear oil. It is a steam cylinder oil. Also, it is my experience in my '36 PU that while "600W" oil is more viscous than readily available parts stores gear oils it is not SAE 160.
I called Lubriplate and they have a product called SPO-288 gear oil. It is a true SAE 160 oil and my '36 shifts better with the Lubriplate than it ever has, and I've owned it since 1965. The trans in my '36 was Chevy's first attempt at synchromesh and it really wasn't ready for prime time. It has large synchronizer cone-cup end play and a very small 3rd gear synchronizer. But it's way better with the Lubriplate product than with any other oil I've ever tried. I first assembled this transmission in the 1960s when parts were still readily available for it. It has all new synchronizing parts and shift forks and the synchronizer end play is set at the exact middle of the factory specified dimension of 0.025" - 0.035".
As for the GL-5/yellow metal argument, in asking lubrication engineers I've learned that for the past several decades GL-5 gear oils have been buffered and are completely safe to use in contact with yellow metals.
