I am about to intstall a 3.73 unit on my truck and was wondering if I need to replace the oil or use the stock one? Some say the original oil works and some say any gear oil is better than the stock. Will the original oil work?
What's up with synthetic?

 

The stock differential oil IS synthetic.
GM recommends using synthetic SAE 75W-90 gear oil (part# 12378261).
If you decide to throw in a posi with the gears, for example an Eaton LSD, then you'll have to use the conventional oil that Eaton recommends plus the LSD additive, it's a friction modifier.
Here's an article on how to change your differential oil.

http://www.gm-trucks.com/IAR_RearDifffluid.shtml

 

what's the best posi unit and how much?

 

I just got the Eaton installed ($350 off ebay) and I LOVE it. I can't explain how much fun it is. Doesn't take long to learn how much throttle you need to give it to break the tires loose around corners, or anywhere for that matter. Off the line it hooks up great.
This is the only one I've had, but I know Auburn makes one, less expensive, but you can't rebuild it unlike the Eaton.

 

are your 3.73s still up for sale. I have a friend interested?

 

Yea, I gave you the shipping costs in my for sale thread. Let me know.