The book says any 75W-90, GL4 or GL5. GL5 is formulated for EP and hypoid gears and can contain sulphur. There is no hard evidence that I've found to prove or disprove that it eats synchros.
Most prefer to play it safe and use Redline MT90. It's 15 dollars a quart, but there's no worry of sulphur, and it shifts nicely.
I'm using mostly Supertech synthetic blend for over a year and 30,000 miles with no discernible damage to my synchros. I say mostly because I have a weeping axle seal and have topped it off with a quart or so of Dino 80W-90 GL5.
Pennzoil Synchromesh is not a bad choice either.