Adam,
I use sythetic gear lube in mine. I have use sythetic's since I spent a couple of years road racing sports cars, a few years back. I first used it in the constant velocity joints on the racecar, many folks were having problems tearing them up. I never in the two years experienced a lubricant related failure. Now missing a shift and over reving the thing (gianed a few extra engine pieces), getting up close and personal to the armco barrier (a few extra body and suspension pieces) I could not blame on lubricants. Except possibly I missed shifts more often in before changing the gearbox over to synthetic lube, it shifted much smoother and faster with the slippery, and granted expensive stuf. Also, if memory servs you will not find a gasket on the differential cover, I believe it is a layer of sealant.
BTW - my birthday is Sept 18 as well!
Les