90 weight straight mineral oil- - - -Tractor Supply sells it as Ford tractor transmission fluid. DO NOT use synthetic lubes of any kind or weight such as 85W-140, for instance. They're too slippery, and will cause gear clashing during shifting. 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!
How is 85w-140 too slippery? It seems the high viscosity would be anything but slippery as long as it isn't synthetic.
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 a note that I have moved your thread and conversation to the Driveline forum as the best place to further discuss this topic.
In respect to your question about what weight oil should be used in an SM420 transmission, that has been discussed quite often. You could use the Search The Site tool found in the left margin to see many of the threads on the topic that already exist.
Otherwise, have you looked in any of the easily found sources like original/reprinted and even on-line copies of owners manuals and Service Manuals???
See below for a screen shot of an "Owners Manual" page concerning transmission lubricants to be used (Note: this was taken from a '48 Chevy Truck Owners Manual, but is the same for your '49).
I also wonder why you asked about using an "additive"???
Hope this helps.
Dan
~ 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)
This is whats been in my 3 speed with borg warner OD for years now. Zero complaints or problems. All original everything and shifts like butter. TSC 90 Mineral Oil
The mineral oil sounds like a good way to go. Pennzoil still sells a GL-3 grade gear oil that would probably work well too. I just picked up a qt of 90 wt at the local Napa store for the M20 Muncie 4 speed in my C10.
1966 C-10 Short Stepside HiPo Step-by-step ~ and on-going resto build in the Project Journals
