Trevor and Dan,
I made a few calls regarding GL-5 gear lubes and manual transmissions.
There is a new specification that is optional to comply with. It is
called: MT-1. You need to find out if your lubricant complies with it.
I talked to the folks at Amsoil, Red Line, and Valvoline and they all
told me the same thing. The MT-1 specification says that any gear oil
specified for manual transmissions must NOT be corrosive to yellow
metals. No matter what the GL-rating. Amsoil 75W-90 GL-5 meets the MT-1
spec.
I also found out that most non-synthetic gear lubes become corrosive
aboue 200° F. Some synthetics raise that to 260° F. Red Line and Amsoil
have lubes that are good up to 350° F. But they meet the MT-1 spec so it
doesn't matter.
Hope this clears up some uncertainty.
Pat in NC
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