Differential Fluid

What is the best gear oil to put into the differential? I have a 2004 2WD with the 4.8 liter.

The GM synthetic fluid. It's really high though.

So...say I were to go to Auto Zone or O'reilys?

Not sure what they have that's equivalent. Someone does though.

I know they sell a sycromesh an additive

Synchromesh is manual transmission fluid.

Whoops! I was meaning LSD additive, but I'm pretty sure mine doens't have that.

They don't need additive if you use the proper $30 per bottle GM stuff.

I just called O'Reilys and they have the 80-90W Lucas and Royal Purple. Which ones better?

i use royal purple..... but thats just a thing that i do... all pluids in my truck are RP..... if its better or not i dont know

Ok, my next question is...do I need to remove the cover and replace the gasket, or can I draw it out and replace it? I've heard synthetics will "eat away" at the gasket material GM uses. True or False? GasGuzzler?

We hardly ever have leak issues on the 8.6" rear axle. The older ones have a drain but I think they stopped that in 2003. If you have more than 100K miles pull the over to clean out the housing better. Replace the gasket. At 50K or less you could get away with drawing it out. That's what I did on my 210K mile Blazer....at 120K.

Royal Purple is the way to go. We use it a work on a 6million dollar Robot and we dropped almost 15 degrees at operating temp. That right there sold me on it. Every think i have has RP in it.

I am running Amsoil in everything. Follow the link. They test everything against there own oils.

AMSOIL Performance Tests

I went ahead and changed the fluid with Mobil 1 75w-90. I didn't remove the cover, I just sucked it all out with an electronic portable suction unit off my ambulance (pretty creative) and refilled it. Seems to be a lot smoother, but it could just be my imagination. I never really paid attention, but is there a drain plug on the transmission? I won't have time to take a look for a few days. I was told my a chevy dealer not to worry about the filter until 100,000 miles and to just drain and refill. I've got a hair under 80,000 now. Is it okay to put synthetic in without getting all the original stuff out?

An 04 probably does not have the trans drain plug. They quit that in 02 or 03. I would not use synthetic trans fuild....Dextron VI FTW. At 80K I'd do a full service with filter.

full service as in a system flush and fill? Dealer gave me a price of $245.00 for that. Sound about right?

I would not flush it unless it failed and was being rebuilt (gets the debris out of the cooler system).

so just a fluid and filter change? Is synthetic bad for the tranny?

I just wouldn't use. I have no proof but I like the Dextron a lot. Others may suggest differently.

I'll stick with the Dextron

Get your wallet out :LOL:

well that doesn't sound promising! LOL