I'm having some problems with my manual BMW 330Xi '01. Underneath the oil cap, I have found this yellow sludge, that I believe is a mix of water and oil and this has persisted for the last couple months. There has been no white, sweet smelling smoke coming out the tailpipe, even though the coolant level did drop half a cup in the past 3 months, so I don't think it's a head gasket failure, but I'm not for sure. Also, I typically drive long distances, so I don't think the excess water could be from just driving it short distances in the cold weather without first properly heating it up. I don't know how the excess water from combustion in the crank case usually exits the engine, so another idea was the PCV Valve. Any ideas as to what could be wrong?