90 weight straight mineral oil- - - -NOT any of the synthetics, multi-weight lubes like 85/140 or "mouse milk" additives, or anything fancy. The gears in a 420 are heavy and have a lot of inertia, and the super slippery lubes don't let the synchronizer blocker rings get enough traction on the gears to match up speed during shifting and prevent grinding. The 90 weight mineral oil Tractor Supply sells for Ford N-series tractors is the correct lube for the 420 transmission.
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!