Regular 90 weight mineral oil gear lube (not hypoid lube) is the best choice. Some of the highly touted synthetic gear oils are so slippery the brass blocker rings in the synchronizers can't grab the cone clutch area on the gears, resulting in grinding gears on a downshift. For the synchronizers to work properly, there can't be any power transmitted to the cluster gear by the clutch- - - -it needs to be fully disengaged. There's a lot of inertia in those big heavy gears, so don't expect a 420 to shift like a sports car- - - - -it ain't gonna happen.

I learned to drive big rigs on a B-73 Mack with a 5 X 3 setup with two shift levers- - - - -both crash boxes with no synchronizers. The only time we used the clutch on that rig was for starting and stopping, and grinding a gear was a rare occurrence.
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!