You'll find lots of discussions in the archives about transmission VS rear axle lube, and plenty of opinions. GL5 lube probably isn't a good idea in transmissions like the SM420 4 speed- - - - -the gears are big, heavy, and have lots of inertia, and GL-5 is just too slippery to let the synchronizers function properly. GL-1 straight 90 weight would be a good choice there. It's also plenty good enough for rear axle lube, plus it's WAY less expensive than the fancy Moose Juice most local auto parts stores sell. In the southeast USA, Tractor Supply sells a straight 90 weight gear lube that's used in Ford N-series tractors- - - - -just like the original stuff Chevy ran in the 1940s, 50s, and 60s.

www.tractorsupply.com/tsc/product/traveller-ford-tractor-hydraulic-oil-2-gal-t806600-1636284

Fill transmissions and rear ends level with the bottom of the fill hole in the housing with the vehicle on a flat floor- - - - -that's the recommended fluid level.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!