Synthetic lube in a SM-420 transmission will be a major mistake. The countergear has a lot of rotating mass, and synthetic lube is too slippery to allow the synchronizers to do a good job of matching gear speeds during shifting, particularly downshifts. Rear axles use hypoid gears that mesh with a sliding action- - - -transmissions don't, so using a general purpose gear lube with hypoid extreme pressure additives in a transmission isn't really a good idea. The synthetic lube in the rear axle won't hurt anything- - - -it might even lower the operating temperature a little. Using synthetic or hypoid-gear lube in a transmission isn't really a good idea. The 90 weight straight mineral oil that Ford specifies for their N-series tractors (8N, 9N, etc.) is ideal for a GM truck transmission.
Jerry
