Oily residue on top left side of radiator cover--what is it?

[LoveTwoCamp]

This is a 2004 chevy diesel truck. Several months ago, I had the truck into the dealer for another issue, and saw this "residue" and asked them to check it out. They said that they did not know what it was, and could not find a problem. They wiped it off.

I was filling the wiper reservoir yesterday, and found more of this oily residue. It looks like oil mixed with water. Nothing is leaking to the ground.

Any ideas as to what it is, or the cause?

[wintersun]

With a 2004.5 with the LLY engine I would suspect the head gasket is bad and have it checked out fully before driving the truck. Known problem with the early production LLY engines and much more expensive a repair if coolant gets into the crankcase.

[recycler]

left hand side as facing truck? I think the a/c lines run near there and have a joint there..when the oil from the system comes out it has that whitish color..