New style oil filler cap. If your cap is leaking, first try cleaning the surface around the gasket. If that doesn't work it may be time for a new cap. The oil cap is actually an integral part of the crankcase vent and vacuum pressure inside the valve cover. Just like a coolant reservoir cap, the oil filler cap is one of the things that regulates the amount of pressure and vacuum inside the engine. Keep in mind that too much pressure can blow oil from the cap. This would indicate a problem elsewhere in the crankcase vent system.
Also, BMW does not recommend placing the cap upside down when it's removed as residual oil will leak down past the seal. Keep the cap right side up on a clean surface.