Is it a creamy white semi-solid goop, collecting under the oil filler cap?

If so, it is from moisture/condensation in the engine, as a result of lots of short trips that don't give the engine time to get hot enough and burn off the moisture. Best bet is to get the engine right up to temperature, and hold it there (on a long drive).Then drain and replace the oil. You may need to do it more than once. The fix is fewer short trips, particularly when it is cold out. The breather can affect it, but unless you give the engine a chance to get hot and burn the moisture off, it will keep happening.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue