The SM-420 is a pretty forgiving transmission, but it's got big, heavy gears with enough inertia that the synchronizers have a problem matching gear speeds when shifting. Stay away from the super-slippery synthetic lubes that might not let them do their thing. The Tractor Supply 90 weight mineral oil sounds like the best choice, and it's probably a lot less costly than the fancy Moose Juice to boot! Use it in the transmission only- - - - -it won't have the hypoid lube additives the rear end needs. The 80W-90 would be a better choice there, or any general purpose hypoid gear lube.
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!