The extreme pressure lubricants intended for use in Hypoid-gear rear axles can interfere with the operation of the synchronizer rings in some transmissions with heavy gears that require a lot of braking to prevent grinding while shifting- - -like the SM-420, for instance. Back when I was doing a good amount of repair on Fuller Roadranger transmissions I could tell which ones had used multi-purpose lube- - - -the synchronizers would be blackened by carbon deposits caused by slippage and overheating of the brass synchronizer rings.
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!