Just thinking about going to syn. in the diffs. The rear diff is a locker and I was wondering if the syn oil is ok for that or if it is compatible with a posi additive. What viscosity of oil would you go to. Thanks

No need for a posi additive. I use Mobil 75w90 synthetic. You can pick it up at any Esso dealer. You can also get the GM grape juice from your GM dealer. Part #12378261 The Gm stuff is pretty expensive.

Why No Additive In Gm Rear Ends...how Long Have They Been Like This?

Ever since they went from posi to a locker.

I use Castrol 75w90, takes 3 litres so price matters. If I'm not mistaken, it should come from the factory with synthetic oil in the rear diff.

The Dmaxes have synthetic gear oil from the factory. Unless it really needs changed, leave it in. It is good stuff.

Thanks for the help guys, my diff needs a change anyway so I will be sure to stick to syn. I haven't changed it since I got the truck (used) so it may have never been changed. Thanks, Doug

they ALL need changed after 500 miles or so.

Change it. You will amazed (appalled?) at the stuff that comes out of there. I've used both Mobil 75W90 and currently Redline 75W140 with no complaints from the rear end.

TC

I use Redline in all but trans

I normally use Mobil 1
I drained at 5000 miles and I agree If you have never drained them (front and rear) you are going to be shocked at the "crap" that is in the oil. Drain them hot.
I used Wally World full syn 75/140 in another application and it looked and smelled just like the expensive GM.

Mobil 75/90 syn.here

Amsoil 75w90, when I drained the factory stuff at 500 miles it was over a quart low.:mad:

I use mobil 1 75-90 syn in the front and rear, good stuff.