Just an FYI as I posted in another thread last evening, I overhauled an SM420 about 2 years ago with a Novak kit. At the same time I switched from GL4 90 lube to mineral 90 from NAPA. I don't recall if it was their house brand. Anyhow, after about15,000 miles, when hot (steady running for a couple of hours) the 3rd and 4th gear synchros are grinding if I shift too fast. This never happened before. I don't know if it's the Novak synchros or the mineral oil but next time I change the gearbox oil I'll go back to GL4 90 if I can still get it.
