Lube for Manual Trans?

I'm rebuilding a 5-speed overdrive trans because the main shaft center bearing started crumbling away. It's an '89 (Mazda) trans for a 3.0L Aero. The fluid was black and at the right level. No loss of fluid. The case is bone dry on the outside - even the rear boot. My guess is that the bearing lost pre-load, maybe because of case deformation. All the other bearings are like new. (Except the plastic clutch bearing.)

I'm gonna' check the endplay to make sure I have preload when it goes back together, but I'm wondering what the best choice for lube is. The shop manual recommends Dexron II for manual trans which is superceded by Dexron III. Would a synthetic gear lube be better? Synthetic motor oil? Would a heavier lube gum up the synchros? Was the Dexron chosen to meet MPG requirements or for cost?