You can get Redline MT90 for about $8 a quart, online, the trans requires 3.6 quarts, so you'll need to buy 4 quarts. No one sells it on the shelf in my area, I order online from a place 50 miles away, I get it the next day or two later.
MT90 is actually GL-4, but the literature about the oil will tell you that the synthetic blend makes it more stable at high temps and it won't become corrosive to synchros until some crazy temp twice what most GL-4 oils do. Many people have used it and NOT reported a problem.
MT-90 or other "MANUAL TRANSMISSION FLUIDS" have friction modifiers that help the Synchros engage better. You want something other than regular gear oil, the friction modifiers make the shifting so much smoother and easier.
ATF has friction modifiers in them, and more and more manual trans use that as their fluid. I wouldn't recommend using anything other than what the manufacturer recommends, unless your willing to accept the risk. BUT, the new Chrysler ATF+4 is now on the walmart shelfs for about $3 a quart under the SuperTech Generic Brand Name. Chrysler is currently using ATF+4 as its factory fill for manual transmissions and power steering fluid. Its synthetic, and I guess its some sort of super fluid that Chrysler developed to finally fix the problems they have been having with the A-604 transmission that has caused them so many problems. Anyone Game? Willing to try ATF+4 for their manual trans? It seems a little thin, although it is synthetic, if you really stress your trans.