FWIW, I've had excellent results running Redline Shockproof Heavy in the gearbox. I think that has the viscosity of a 90W-140 and protects like a 250 weight gear lube. GL-5, for those still afeared of that bogeyman. I wouldn't use anything else in any of my gearboxes, totally sold on the stuff.
I use Redline MT-90 in the primary chaincase. That's a 90W and I've never had a bit of clutch slipping or sticking (I do kick through a couple times with the clutch disengaged before starting) and I really like the thought of that high quality lube in there. FWIW/YMMV.
No affiliation with Redline, but I love their products. Are they a private company? Cause if not, I wouldn't mind buying a few shares too...