I have always used what was recommended in my workshop manual and measure the right amount that goes into my GB with a measuring cup, has worked for me for the last 48+ years, been so long since I been in my GB, since replacing the worn kick start gear, pawl and a broken kickstart shaft where the pawl was connected to the shaft, caused from the worn gear, haven't had had any problems since, that was well over 20 years ago, the layshaft bearing blew back in 1978 with 12k miles on the clock.
Hypoid GB oil 90w is what I have always used and only replace the oil when I go inside my GB.
Ashley