I recently asked for similar situations or suggestions regarding an oil leak on my 2003 Tahoe LT, 5.3L. I took it to the dealer and asked them to take a look. They degreased the oil pan and ran it for a while. They said it was now dry and blamed "assembly lube". I told them I was suspicous of that verdict seeing how I had gone 9k miles without that happening. I could understand if it that was the cause in the first couple of thousand miles. The response was, you're dry and good to go.
The next night after my wife had driven it around town, I looked again with a flashlight and it was wet with oil, just like when I took it in the day before. I snapped a picture with the digital camera so the service advisor could see the scope of my problem since they don't ever take a look. (I know, it's not there job to.) They kept it and called me later that day to say they were putting a whole new oil pan and gasket on it. They said both were leaking. So far, no more leaking, but I'll keep an eye on it for the next month or so just to be sure.