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Apr 26th, 2026 | | AD Chevy Trucks  Over 6,000 pictures Brad Allen has an awesome collection of Chevrolet factory pictures that he has set up from film strips. Check them out!
Lots of work on Brad's part ... pure enjoyment (and references) for you!
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| | | | Joined: Jan 2025 Posts: 80 'Bolter | | 'Bolter Joined: Jan 2025 Posts: 80 | 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. What does gm recommend ? | | |
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