I've been running Redline MT-90 (which is a synthetic GL4) in an A-type OD gearbox for over 10 years now (almost as long as Geo has). Works fine. The MT-90 is specifically friction-modified for use in synchronized transmissions; it actually improves synchro operation over conventional GL4. Kind of expensive and not usually found in the corner FLAPS, but the improvement is worth the effort and expense, IMO. Prices vary a lot, so it pays to shop around a little. I eventually bought a case of it on Amazon but their prices change frequently so it may be cheaper somewhere else today.
Oh, BTW, at the risk of being "arrogant" again, I'd like to point out that GL4
is a "hypoid" oil. In the US it's known as "extreme pressure", but that's just another example of two countries separated by a common language.