I read this article by Jay Leno a while back in Popular Mechanics:

"Shifting with old transmissions can also be an issue. Red Line Oil makes a product called MTL 70W80 GL-4, a manual transmission lubricant that acts like a liquid synchronizer. My 1972 Dodge Challenger was always difficult to get into gear, and the gears ground when I shifted. I added a quart of MTL, and it doesn't even feel like the same transmission. MTL provides the perfect coefficient of friction; it protects the gears and the synchronizers. Better shifting through chemistry? You bet."

Might be worth giving a try. I'm not sure what the price is though. How much fluid does it take to fill the box?