What fluid for a 4-speed trans?

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???