90 weight gear oil and 50 weight engine oil are the same viscosity, the numbers are different to avoid misuse.On the websites of some major lube makers is the warning about using modern GL-5 gear lubes in older gear boxes,additives attacking yellow metals(brass and bronze?) as mentioned above.If the gear box was designed to use motor oil,or a light oil then it should be ok.Spicer used to recomend the use of 50 weight motor oil ,not gear oil in some medium duty truck trannies.Many modern gear boxes use thin ATF as a lube.The NP 3500-4500 series transmissions use a light gear oil.The NP A833 4 spd OD tranny is supposed to use ATF, but Mopar guys say to use a 75-90 gear oil for less noise and longer life.
