My experience:
1. factory fill - swapped at 11k miles and it felt like a new truck, I gained some power.
I used synthetic 75w90 without any addy, I think I used mobil 1.
I will do it again in another 20k miles as I did not really get a chance to break in my rear end, the duramax guys say, tow a 15k trailer for few thousand miles and around the city and you are good.