Oil having a gasoline smell is kind of the norm. If you were washing down cylinders bad enough to have gasoline truly mixing with the oil you would know have smoke billowing out the tail pipe. As far as the red color on the gas cap, there's no way dex cool has made it to your gas tank, the two system have zero places where they can feed each other. The redish color is more or less the plastic being discolored with age. As for leaks, these motors in my experience dont leak much at all. Most common place I've seen 4.8s and 5.3s leaking is the rear main seal, however, sometimes it wasn't the rear main at all, it was the oil pressure sensor/switch on the back of the block leaking down. Hope this helps.
