[Batman]

I would use 10W-40 conventional for the break in. Then change it, run regular 5W30 for the first few hundred miles, them switch to full synthetic.

DO NOT PUT SYNTHETIC IN A T56 !!!!! Royal Purple isn't supposed to be used in a T56, I had it in mine and it killed the synchro's (according to my friend who rebuilds them for a living anyway, that was the cause of death) Turned a brand new T56 to junk in about 400 miles. I know a few people who haven't had problems but according to my friend and GM it is supposed to be regular dexron III

For a stock rear I would use synthetic fluid, shouldn't use it in a moser ( I believe it is an Eaton Posi thing...)

Valvoline also has a synthetic brake fluid now, it is marginally more expensive then regular (like $2-$3 a bottle) but it is supposed to be water resistant so it won't pick up moisture. I have it in my car now but haven't had alot of driving time on it so I can't give you a real good opinion on it yet.