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.