The most common cause of oil fouling on a spark plug is caused by badly worn piston rings or cylinder walls within the automobile's motor. Oil can also be pulled into the chamber because of excessive clearance in the valve stem guides.
Oil fouling around the top of the spark plug can also be caused by badly worn valve stem seals. If a PCV valve is inoperative or plugged, there can be a buildup of crankcase pressure, which can force oil vapors and oil past the valve guides and rings and into the engines combustion chamber. It is important that an engine run at the right temperatures while also using the proper grade of gasoline in order to avoid any issues with spark plug fouling.