Mike, I use Wal-Mart full synthetic, GL5. Use this in the rear differential as well.
The older transmissions call for GL-1...GL-5 is for the rear and has EP (Extreme Pressure) additives which can attack the yellow metals in the trans (synchronizers), I don't know if synthetic attacks yellow or not.
One other issue with synthetic is it's very slippery and won't allow the gears time to slow down between shifts causing the gears to grind.
Mike B
