Just to add to AMK's problem, I had a 91 525 M50 doing the same thing.
Same symptoms, AND overheating, froth under the oil cap and white smoke bellowing.
No oil contamination nor coolant contamination and 3 NON BMW mechanics telling me i
had a bad head gasket.
PITA for near 2 mos so I hardly used the car...
I called a buddy over. European car mechanic. Bled my system with the heater controls
on MAX DEFROST for reasons I can't fully explain and within an hour no more problems.
He told me when there's air in the system that HOT SPOTS can develop; areas where air
gets trapped and can burn off coolant, cause coolant loss and overheating.
His first and last advice... These cars are notorious for air in the cooling systems!
BLEED! BLEED! BLEED!