here my 2 cent im in NC and in the winter my cap looks like that too. my coolent never goes down, and my oil when i change it never looks like that cap just reg oil. i havent had no probs. dont lose to much sleep over it

 

I'd bet its just condensation as well. My oil cap on the LS1 in my GTA looked like that the other night. I've had it idling more than a couple times in the past week or so and its been very cold here. I cleaned it all out of the cap and filler neck. I also checked the oil and everything seems ok... nothing in the oil. Coolant level was a little low but that's because I have a water leak right now. I'd bet your ok... check it all out to make sure... mainly an oil change and pressure check. You'll know right way

 

oh noes...that looks a little too thick to be a simple problem...Don't check anything, just buy a new shortblock, a set of heads, cam, and all the boltons...Now is the chance to make it so your car is better