Check and see if it is the consistency of petroleum jelly. Usually white/cream coloured, sometimes yellowish. You may be able to find it under the oil filler cap as well.

If so, it is moisture/condensation, emulsified with the oil, and a sign that you aren't running it long enough/hard enough to burn the moisture off. It leads to more engine problems, including potential failures of the crankcase ventilation valve/oil separator. Lots of posts on the subject.

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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