My 96 3500 SRW 6.5TD needs a rear axle oil change. The owners manual says if its a Dana, use 75w140 synthetic... if it isn't (which I do not think it is), use 80w90. I know I have 4.10's and a limited slip. My next question is if it is not a Dana, is it alright to use the Mobil 1 75w90 with the LS additive already in it that Mobil sells? Lastly, how much oil do I need? Thanks!

1. Check out this thread. It discusses the identification.
http://www.dieselplace.com/forum/showthread.php?t=349010

2. TSB 99-04-20-001 from GM says it's o.k. to use full synthetic 75W-90 as long as you don't have RTV. If you google that TSB number, you should be able to find someone that posted it.

3. Hope that helps.

Good luck.