GL-5 is for modern transmissions with small diameter, lightweight gears. Transmissions from the 1950's and before have big gears with a lot of weight and inertia, and running any kind of synthetic lube is absolutely the worst possible choice anyone could make. Add the extreme pressure additives used in rear axles with hypoid gears to the mix, and you've got a recipe for duisaster. "New" isn't always "better"!
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!