When was the last time you had an oil change? My pre-owned Jeep Grand Cherokee had the same issue when I bought it. I've always made it a point to change the oil in my vehicles every 4500 miles (7000 kms) or so and the Jeep has never had this problem since. My 335i oil cap is as clean as a whistle too.
07 335i e90 ► TiAg w/ Black Int ► Sport/Premium/Winter► Michelin PSS► Blackout Trim► Matte Black Grille►LUX v5.1► 20% Tint► Logic 7► HPB Interior LEDs