What fluid for a 4-speed trans?

harrisonm said

OK, my mind is blown! When I rebuilt my 833 about 10 years or so ago, I used 75-90 because I thought that was what we did. I have never heard of using motor oil (20-50) or ATF in an older 4 speed. I know that some modern manual transmissions use ATF, so I guess I can understand that. But is it really OK to use 20-50 motor oil???

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Yes. There are probably several better lubes to use in MT applications, but 20W-50 or even better straight SAE 50 engine oil has been used for zillions of miles of service with zero of issues. A grade 50 motor oil has an equivalent viscosity to a SAE 90 gear lube. Probably just as important as viscosity is a manual transmission lube's compatibility with yellow metals. Hence, using a GL-4 or a modern MT lube is the best move today.