I'm about to change both the front and rear differential fluid in my 05' GMC 4x4. I know the rear already has synthetic and the front is petroleum. Now I got under the truck and looked at the front diff "Vent Cap" , here's where I'm a little confused. The plastic cap that's inserted or screwed into the front diff. is white and from there it goes to a little black vent tube up into the engine compartment and on the end is a little black vent cap. So do I already have the white vent cap that is compatible with synthetic. Or is everyone talkin' about the vent that's on the end of the tube up in the engine compartment ?

the cap is on the diff.. sounds like your all set

Are you sure the front end is filled with conventional petroleum? I seem to remember a post by Eric last spring saying GM made a 2004 mid year assembly line change and started putting synthetic in the front ends. Also, the original lube in the front of mine sure smelled like GM "grape juice" when I changed it.

Here is a TSB that may have motivated the change.

http://dieselplace.com/forum/showthread.php?t=515

Well I know when I heard the factory were sending these things out low on fluids, I took it in to have all the fluids topped off and they told me the rear diff took the syn. and the front diff. took the petroleum based stuff. Now this came from a tech at the dealer. I sure hope he knew what he was talkin' about other wise I'm glad I'm about to change the fluid in them.

Straight from my '05 manual: Rear Axle SAE 75W-90 Synthetic Axle Lubricant
Front Axle SAE 80W-90 Axle Lubricant

It doesn't specify synthetic for the front

Yeah exactly I also read that too. I'm changing the front to synthetic.