Oil Separator Freeze Up
This is a common problem, well documented on this and other forums. BMWNA is very aware. Mine failed last year and the fix was $5500 Cdn. I negotiated with BMW and we each paid half.
A little history lesson I learned doing some research...because this only happens in approx 1% of the overall world market, BMW does not consider this to be an issue. However in the last 4 - 5 years of production this problem has gone away. This problem part was "re-engineered".
There are two senarios: the vent freezes open or closed, blowing oil out the vent or blowing past your head gaskets. Both lead to catastrophic engine failure, meaing replacement of the engne. Very costly!
There are 2 possible repairs. 1) a type of "heat tape" wrapped around the parts or 2) a complete re-engineer of the parts, which means relocating some of the plumbing. This is what I had done to mine ('03 4.4i sport). The heat tape may not always work. Especialy up here where it gets realy cold - Alberta Canada.
A piece of advice I got from my aftermarket mecahnic...place some cardboard inside the hood in front of the radiator when it gets real cold. This keeps the engine bay warmer and helps eliminate the problem.
If they are in fixing this problem, they may recommend a "valley pan" gasket replacement. I'm told these sometimes wear out around 120,000 Kms. I had mine replaced at the same time. The top of the intake is removed to complete the freeze up repair, so this reduces the overall cost if you have to go back and get that done later.
We stll love the vehicle. Didn't consider trading when the problem showed up. No problems since that reapir.