You mean "curious butt concern" hah I funny. I'm gonna assume this is on the driver side. Make sure the vent that goes to the intake from by the oil fill isn't disconnected, leaking etc it could leave that film, could be a leaky manifold gasket, i use flammable brake clean with a straw, fire the truck up and start squirting around the gasket areas. If it revs up its leaking!