Quote:
Originally Posted by ChrisSlicks
Generally speaking you shouldn't use GL-5 oils in the manual transmission (in the diff is fine). They contain pressure additives such as sulfur than can cause corrosion of the brass components, and are sometimes too slippery for proper syncro engagement. This Motul formulation might be okay, you should do some research.
Personally I was just do the diff right now (it gets nasty quickly) and do the transmission when you can get your hands on the new Redline MT-85.
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From what I understand most GL-5 rated gear oils are backwards compatible to a GL-4 rating. But you are right, in watching out for sulphur based additives, as they are not yellow metal (brass/copper) friendly.
One ASTM test to look for, on a gear oil, is the corrosion test with a result of 1 or less.
FYI my choice of gear oil is Shell Spirax synthetic 75W90 gear oil. I've been using it for 5 years now on all my vehicles, with great success.