75W90 is the proper weight. I'd recommend Redline MT90 (not MTL90) or their super light-weight shock proof stuff. You may want to check out
www.myoilshop.com for some info on Redline. The drain plug is on the right-hand bottom side of the tranny. It'll take a 1/2" ratchet and probably a rubber mallet to get the plug out. The fill plug is 3/8" and on the front of the tranny about half way up in the middle. I'm considering the side facing the radiator to be the front.
You may experience some leaking after you switch to synthetic. The reverse and/or neutral switches may leak or you could even experience a leak from the main gasket that holds the two halves of the case together. I've experienced both and it isn't much fun taking the tranny apart to re-seal the case. The switches aren't too much trouble and you may want to change them before hand. Make sure the bolts that hold the case together are tight before filling up with synthetic.
BTW, what is the name and number of the place that you had your cam reground at? Thanks.