I just changed the oil in my M21 to red line 75w-90 gear lube, GL5. However, I've heard that I should have used a GL4 lube because the GL5 can actually harm the synchro's. Has anyone heard about this?

Any lubricant with sulfur will attack the brass syncro's. A hi-impact gear lube uses sulfur as the 'hi-impact' additive. Hence the 'throw-away-the-shirt' smell from getting into it while working on the car.

For a car that sees a lot of heavy duty action the lower syncro life is offset by better gear durability. With a yearly teardown to freshen things up it's no big deal.

For a street driven car that see few miles and isn't constantly being raced (did I really just type that?), I prefer a lube that does not have sulfer in it. Such as ATF, MTL, GM SyncroMesh oil. . .

Bob.

Thanks Bob. I looked at my Red Line bottles, read the tech stuff on it's site, and discovered that I'm okay. I used the 75w-90 MT NS, which, I assume, means "non-sulphur". Everything that I read on the Red Line site says the lube is okay for manual trannys.

I'm glad, because I didn't want to end up changing the fluid again.