Hey guys, I changed the fluid in the front diff today to mobil 1 synthetic. When I drained it, the old fluid was a mustard yellow. Is that normal? I've never seen any diff grease this color...

Also, is the rear as simple as unscrewing the plug in the bottom, or does the cover have to come off? Thanks.!

I'm thinking the front diff. must of had water in it. Yes on the rear diff. Pull plug and drain. Mobil 1 syn 75-90 good choice....Rick

Well, that's what I was kinda thinking, but I didn't know on the color. The truck only has 3500 miles on it. How the heck would anything get in there to contaminate it? I was hoping maybe GM's fluid was yellowish.....

Thanks for the info on the rear. That's tommorrow's project.....):h

What you found is normal. Don't panic.

I think if you do a search, you'll find that it something along the lines of stuff from assembly to cheap gear lube.

In any case, we all have found the same on all of our trucks and Eric and the boys said "No Problem".

Put a couple of quarts of Mobil 1 75W-90 in it a call it a day.

Thanks guys..... i did a lil more searching and found the answer that it's gear marking paint or something that colors it... made me feel better to get it outa there and get some synthetic in. I was suprised how much crap was on the magnetic drain plug.

I will say, this is the easiest truck to service I've ever had. Everything has drain plugs and is easy to get at and doesn't drip on anything but into the drain pan. FINALLY! Good job engineers.!

Its all good, thats perfectly normal.

That weird yellow stuff is what they put in at the factory, why do I know this - I changes all fluids to synthetic the week I got mine and it never hit water.

Its residue from the gear marking compound. When is mixes with the gear oil, thats what you get. Mustard

Changed the rear today.......boy, that stuff was nasty also. Black as tar. This truck has 2300 easy miles, and 1200 pulling a 10,200lb boat. It certainly didn't look like the clean 'grape juice' that went back in. My advice to anybody that hasn't changed theirs, CHANGE IT!