Rear end - synthetic fluid

[Ozzie]

I just swapped out my stock 4-lug 28 splines for a set of dutchman 5 lug 28 splines. I have a 3.73 posi-trac rear end and I was wondering if I should go with a synthetic rear end fluid, and if so, what? I was leaning towards an 80W-90 maybe in a royal purple or lucas oil. Can anyone tell me the correct fluid to use?

[FATHERFORD]

i always used the light weight royal purple stuff.

[anthonydalrymple]

...or Mobil1 synthetic is good stuff as well. IMHO... the synthetics do not require the use of the traction additive....(read the bottle on the Mobil1 to verify).