Real truck transmissions have big gears with lots of inertia. The super-slippery synthetic lubes don't let the synchronizer rings get a good grip on them to match gear speeds as a shift is made. Straight mineral oil such as the Ford N-series tractor transmission lube sold by Tractor supply is a good one to use, plus it's a lot less expensive to buy. I believe it has a "GL-1" rating. 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!
I switched to NAPA sourced SAE90 mineral oil on my SM420. I will probably switch back to GL4. At higher RPM, say around 3,000 RPM, the 2-3 and 3-4 shifts have begun to clash when hot (running at freeway speeds for a couple of hours) about 10,000 miles after overhaul, particularly the 3-4 shift. This was not a problem with GL4 oil.
For the New Process 5421 (I suspect this is a 542L) what does GM specify? Use that. Also be aware that additives with chlorinated or sulpher compounds will attack brass and bronze parts. I ignored this caution once and paid the price. I believe some synthetic gear oils will also attack copper alloy parts.
My Spicer/Brownlipe 6231A says use SAE50 engine oil. I've not been too careful about what I put in it since there are no bushings or other brass parts. Some oils in that box are quieter than others and I do note that it's noisier when plumb full and also leaks more. so, I run it about 1" below the fill plug.
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.
I switched to NAPA sourced SAE90 mineral oil on my SM420. I will probably switch back to GL4. At higher RPM, say around 3,000 RPM, the 2-3 and 3-4 shifts have begun to clash when hot (running at freeway speeds for a couple of hours) about 10,000 miles after overhaul, particularly the 3-4 shift. This was not a problem with GL4 oil.
For the New Process 5421 (I suspect this is a 542L) what does GM specify? Use that. Also be aware that additives with chlorinated or sulpher compounds will attack brass and bronze parts. I ignored this caution once and paid the price. I believe some synthetic gear oils will also attack copper alloy parts.
My Spicer/Brownlipe 6231A says use SAE50 engine oil. I've not been too careful about what I put in it since there are no bushings or other brass parts. Some oils in that box are quieter than others and I do note that it's noisier when plumb full and also leaks more. so, I run it about 1" below the fill plug.
How do the synchros work? Not designed for syn. oil. Just straight min. oil.
Ed
Very well actually but if it should have dinosaurs that’s what I’ll get . The oil has only been changed in the transmission twice in the ten years since I’ve been around the truck The first time I changed it, the truck belonged to a guy I worked for had to scrape out gear teeth shards our the bottom of the case from the years of abuse and super truckers floating gears or trying to I should say. I bought the truck five or 6 years ago and I decided I had better change the trans mission oil again this time when I drained and took the cover off there wasn’t any gear teeth I also added a filter to transmission
SAE90 EP GL4 is what I used in my 4-speed SM420 for decades. The synchros were slow but didn't clash. When I switched to Mineral oil as I've said then they began clashing when hot on my recent trip to AZ. I'll go back to the GL4 soon but it's way too cold here for a good test right now, -14 last night. I'll wait for spring.
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.
