Originally Posted by Jeff@topgearsolutions
Only thing you can really do is take off everything and clean, then look again in a few weeks if it's not obvious enough.
I noticed most 335i's with high mileage leak at the oil pan gasket, oil filter housing gasket, and even some oil drips by the turbo and inlets sometimes.
The oil filter housing gasket is very prone to leak as well as the filler cap. The filler cap would be obvious as its on top of the engine, so its probably one of the things Jeff mentioned.