As others have said, it's most likely condensation in the oil. Happens when the oil temp is not getting up above 212* F during driving before turning the engine off. An easy test is to wipe off the cap and then go drive for a while to warm up the oil to the normal operating temp (230-240). Then check the cap as soon as you park it. Should be clean still. The cap will be worse on some days then others depending on the ambient humidity and how often you drive then car the previous day. This is all pretty normal for cars that drive "up north".... I lived in North Dakota for 6.5 years and dealt with every winter.
'07 335xi, Alpine White
| Cold, "Sport", and Luxury packages | Tinted | OEM Black Lines | JB4 | DCI | ER FMIC | Forge DV's | Custom exhaust with secondary cat delete, Magnaflow mufflers, black Dimisa Tips | Staggered 18" V718's, Matte Black | Flappy Paddles | Titanium CF Trim Wrap | BMW Performance Shifter and E-Brake Handle |