On some of the BMWs made in the last 20 years, the crankcase and/or valvecover vent system fails. The result is that the crankcase cannot vent and oil forces its way out the easiest gaps. If this is the case and you seal up the oil cap really tight, expect trouble eventually at less easy to remove and service engine seals.
Always think about why the problem is occurring. Is this happening on 2 year old cars with 20k or mostly 4 year old cars with 40k?
I have not studied the crankcase breather system on this car yet, but I have replaced them on E36 M3, E39 528i and E46 325i. Hopefully, BMW has improved them.