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)
Last edited by Phak1; 01/12/202511:34 PM. Reason: Typo
Phil Moderator, The Engine Shop, Interiors and Project Journals
1952 Chevrolet 3100, Three on the Tree, 4:11 torque tube Updated to: ‘59 235 w/hydraulic lifters, 12v w/alternator, HEI, PCV and Power front Disc Brakes Project Journals Stovebolt Gallery Forum
Definitely DO NOT use the super-slippery synthetic transmission/rear end lubes. The inertia of those big, heavy gears will cause grinding because the synchronizers can't get a grip and match the gear speeds during a shift. 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!
NAPA SAE 90W mineral gear oil or Tractor Supply's Traveller Tractor Mineral gear oil ISO 90.
Craig
My '50 Chevy 3100 5 window, '62-235cu, 3:55 rear My truck ....... Respect The Rust If I'm not working on my truck, '65 m00stang or VW camper, I'm fishing with the wife or smoking Salmon.
NAPA SAE 90W mineral gear oil or Tractor Supply's Traveller Tractor Mineral gear oil ISO 90.
Tractor Supply. It's half the price of the NAPA version.
1950 Chevrolet 3100 (Ol' Roy)
1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1964 Ford Galaxie 500 coupe |1966 Ford Mustang (Little Red) | 1969 Pontiac GTO (The Dishpit) I 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)
Just an FYI as I posted in another thread last evening, I overhauled an SM420 about 2 years ago with a Novak kit. At the same time I switched from GL4 90 lube to mineral 90 from NAPA. I don't recall if it was their house brand. Anyhow, after about15,000 miles, when hot (steady running for a couple of hours) the 3rd and 4th gear synchros are grinding if I shift too fast. This never happened before. I don't know if it's the Novak synchros or the mineral oil but next time I change the gearbox oil I'll go back to GL4 90 if I can still get it.
1951 3800 1-ton "Earning its keep from the get-go" In the DITY Gallery 1962 261 (w/cam, Fenton headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.
