If you think that the residue is oil, check your PCV system. There is a hose from the valve cover to your air cleaner. The vapor from the engine is sucked into the air cleaner and thru the carb to burn the oily vapor. If you have a stock (Chevy) air cleaner, there is a small, coarse filter inside the air cleaner that allows the oil to condense from the vapor. The PVC system is most likely the source of the oily film.