This problem is specific for the 330 model, with the M54B30 engine. A handful of these have a rare problem in which the valve guide seals fail and oil leaks in to the combustion chamber to be burned. Signs are a ton of smoke from the tailpipe that smells like burnt oil when you first start the car in the morning, and lots of smoke from the tailpipe and loss of power under hard acceleration. Costs about $4000 to rebuild the head with the new valves and valve guides. and no, its not the crankcase vent valve, although that was replaced twice prior to this due to a vacuum leak. Long story short, this car is plagued with maintenance issues in general once you hit over 150k and most of them and manageable, but this valve guide seal issue is more serious.

No signs that this was going to happen... one day I was driving my friend and his pet cat to the vet and my car lost power and I heard weird popping noises, almost like a backfire and I see a huge cloud of smoke behind me. This happened every time I accelerated and also when I first started the car in the morning. I always maintained my car really well and dumped a lot of money into caring for it. Did a leak down test and eventually tore into the head and had it rebuilt. Car was fine after that, but at that point, the car was worth only like $6k anyway so I was upset at spending all that money. I got rid of it a year later.... I do miss the car sometimes. It was in mint condition and that was the biggest problem it had.