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BUSY BOLTERS Are you one?  The Shop Area
continues to pull in the most views on the Stovebolt. In August alone there were over 22,000 views in those 13 forums.
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Jul 21st, 2025 | | | | Joined: Feb 2005 Posts: 765 'Bolter | | 'Bolter Joined: Feb 2005 Posts: 765 | Does anyone use the same gear oil in their manual transmission and differential? I have a 4-speed manual and saw that I should not use GL-5 gear oil, but GL-5 would be OK in the rear differential. 90w mineral oil was suggested for the trans, but I'm thinking about using the same for both if I can. I see that NAPA has GL-1 gear oil and thought I could keep it simple and use it for both?
~ John in Utah 1946 1/2Ton w/4-speed manual transmission w/1960 235 engineHere We GoFollow in his DITY Bay- If you think about it, it has been one year ago today!
| | | | | Joined: Apr 2007 Posts: 1,002 'Bolter | | 'Bolter Joined: Apr 2007 Posts: 1,002 | If you have the original 4 speed (reverse gear lock-out lever, non-synchronized) you can use GL-5 gear oil. If you have a later SM420 4 speed tranny you should use only mineral oil because the GL-5 is too slippery for the synchros and additives cause corrosion of the synchros.
Kent | | | | | Joined: Oct 2021 Posts: 5,684 'Bolter | | 'Bolter Joined: Oct 2021 Posts: 5,684 | Kent, he's asking whether the 90 wt mineral oil can be used in the rear axle.
1950 Chevrolet 3100 (Ol' Roy)
1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1956 Cadillac Sixty Special Fleetwood (The Godfather) | 1966 Oldsmobile Toronado (The Purple Knif) | 1966 Ford Mustang (Little Red) | 1964 Ford Galaxie 500 coupe | 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)
| | | | | Joined: Apr 2007 Posts: 1,002 'Bolter | | 'Bolter Joined: Apr 2007 Posts: 1,002 | Otto, you are right! John, I don't think mineral oil can be used in the rear axle. If you want to use one type, use GL-5 in both.
Sorry for the confusion,
Kent
Last edited by Lightholder's Dad; 12/14/2024 8:00 PM.
| | | | | Joined: May 2015 Posts: 9,830 Housekeeping (Moderator) Making a Stovebolt Bed & Paint and Body Shop Forums | | Housekeeping (Moderator) Making a Stovebolt Bed & Paint and Body Shop Forums Joined: May 2015 Posts: 9,830 | For a non-synchro tranny, I'd use GL-5 in it and the rear axle. You can get that stuff ANYWHERE. Not necessarily so with mineral oil.
Kevin 1951 Chevy 3100 work truckFollow this saga in Project JournalPhotos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together. | | | | | Joined: Feb 2005 Posts: 765 'Bolter | | 'Bolter Joined: Feb 2005 Posts: 765 | Yes, I have the 4-speed with reverse gear lock-out lever. Thanks for the input guys!
~ John in Utah 1946 1/2Ton w/4-speed manual transmission w/1960 235 engineHere We GoFollow in his DITY Bay- If you think about it, it has been one year ago today!
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