rear differential fluid -- another question

[2fast2]

Hi. Yes, I searched the archives, but I still want to "make sure" before I do something I might regret.
I've got 6K on my truck now and I've read here that I should've changed the rear differential oil at 500 miles if I did a lot of towing. So far I've only towed much of a load (8-9000 lbs) for a hundred miles or so.
Should I change the fluid? Is it the general consensus that Mobil 1 75W90 or Amsoil synthetic gear lube is OK for the stock locking differential? I'd buy the GM fluid if it was necessary, but since I already hate the local dealer and don't want to give him another dime, it means I have to drive a while to plunk down my hard earned $ for the GM fluid. I'll do what's right, but as inexpensively as possible. Oh, I do limited towing, but love this for highway cruising and trips.
Thanks,
Jim
2003 Silverado 2500 HD D/A CC

[Hound]

Yep, the Mobil and Amsoil stuff is just fine for the front and rear diffs. You may look into Redline too. If you're into heavy towing then the 75-140 would probably be worth a look. If you change the front to synthetic, look to see that you have the white vent on it. The black vent is supposed to be incompatible with synthetic. smile.gif

[2fast2]

Thanks Hound.
I've used Redline in my MR2 and I think it is good stuff too. I like Mobil 1 because it is easy to find locally. Amsoil is also good stuff and sure seemed much less prone to thickening with colder temps. I don't tow that often, so I'd probably be better off with the 75W90, right?
Thanks,
Jim

[JimWilson]

The 75-140 is for the heavier tower amongst us. If you're only towing occasionally, or tow something not terribly heavy, then the 75-90 is just fine.

[Tim Richardson]

I went ahead and changed mine at 5000 miles. I had only done about 100 miles of towing, but I figured I would rather be safe than sorry. When I did drain the rear end, it looked pretty bad (maybe it was supposed to :confused: ) so I didn't regret doing it. Also I went with the Mobil 1 synthetic. Much cheaper than the gm stuff, and meets the same specs.

Tim

[VA_Dmax]

Ok I went and ordered 4 quarts of the GM 75w90 stuff from GMpartsdirect.com and got the stuff in a box today. And you guys werent lying when yall called it grape juice. It had 3 bullets on the front of the bottle and they read like this:

-Superior Oxidation and Thermal Stability
-Long Fluid Life
-New Grape Scent

No kidding GM really puts a bullet on the front of the bottle saying it smells like Grapes. I dont know I guess its a big selling point to have an oil that smells like grapes?

[JimWilson]

It only smeels like grapes before you put it in. When you drain the used stuff out you certainly won't think that smells like grapes. ;)