I have been suffering from a slightly oily smell inside the car for about 4 months. After looking really closely under the bonnet I finally traced it to a broken off return (PCV ?) valve which is attached to the top of the engine.  As the valve is underneath a larger plastic part of the intake system it not that easy to see.   As I pulled the pipe out I saw the broken part and traces of oil in that area, in the pic you can see the small broken valve casing outlet tube.