Could be residual oil from the PCV inside the intake and under "normal operation" (IE, with a cat), it wouldn't show up. I'm going to be plumbing in an oil catch can hopefully in the next week or two (if I get the time and can figure out a place to put it :icon_razz), so that might help with the issue as well. Does your car actually burn oil? Do you have to add a bunch of oil between oil changes?