The paste is a result of moisture mixing with the oil. If the vehicle is running fine (indicating no leaks in the head gasket, etc) then it is likely due to condensation. This results from short trips that don't get the vehicle hot enough to boil off the condensation. Over time, it builds up and is visible under the oil filler cap.

I would do an oil change, and try to drive it such that it gets hot more often. The conditions that lead to this build-up are the same as those that lead to problems with the oil separator/crankcase ventilation valve, since the moisture freezes.

I don't think your choice of oil caused it as much as your driving conditions.

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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