I have seen questions regarding yellow front diff fluid (including my own), but did not see many suggestions as to the cause of the color. Here is the response that I got from my GM mechanic friend...

You just work the piss out of that duramax. It's just not designed for that kind of extreme duty to and from the grocery store two miles every few days. Ha ha!!!!! Not what you wanted to hear? Lot of trucks look like that. Seems sometimes they get carried away with the yellow gear marking compound when the ring and pinion tooth pattern is checked.


Sounds like a reasonable explanation. As you can tell, he likes to give me crap for only using my truck as a "truck" a few times a year.

Here is a pic of the fluid...

http://dieselplace.com/forum/attachment.php?attachmentid=3737

The first time I had mine changed out it was done at the dealer. The oil that came out was what I would call more of a mustard color, and cloudy looking. The tech told me it was a mixture of diff oil and matching compound used when the diff was assembled.I had them switch the front over to synthetic. After another 15K miles I drained the diff again and this time the oil was much cleaner looking. I beleive this is common to find this with these trucks.

I too understand this to be common. I know in some brand new lawn mowers they use this mustard color oil to assist in breaking in the motor. I would assume the reason the front diff is the same color is to help with break in. I don't believe it is water or any other harmful material. I doubt you will have a problem.

When I changed my diff fluid out the first time (after a 500 mile break-in change I had done at the dealers) I had a lot of yellow particulate "mung" in the oil. It's a compound used in assembly and normal for the DMax diff. I didn't know that it was a matching compound, but it looks like you can get a lot of it in the diff depending on the amount put in in assembly and how much gets "washed out" by use prior to the first changes. I put in Amsoil 2000 synthetic and have seen no more yellow goop in the last two changes - just pretty blue fluid.

It sure scared the crap out of me. Mine was the same color as mustard and milky to boot. Now its got nice fresh fluid and will be changed again at 2000 just to see if everything looks better this time