So the only response , for our Canadian friends in all those is Mark's
"have been using fully synthetic GL5 gear oil in my Spitfire's differential and transmission for 8 years now with no ill effects. I do look for one that stated that is it compatible with yellow metals (just to feel more safe).
I have looked into the API service designation for GL5 and it does state "...protection against oxidation, thermal degradation, rust, copper corrosion...""