dougs - the Shop Manual calls for the lubricants shown in the below extract from the 1948-51 Chevy Truck Shop Manual are what is specified for the SM420 4 speed transmission. No additives are called for. Here is an older thread on SM420 gear oil that provides some added info. Hope this is helpful.
~ Dan 1951 Chevy 3 window 3100 Follow this story in the DITY Gallery "My Grandpa Carl's Truck and How it Became Mine" 1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver) US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)
