I have an '01 330Ci, 136k miles, that is using an excessive amount of oil. I was using BMW 5w30, but switched to Castrol 0w30 hoping it would help and I the CCV has been changed. Nothing so far has helped. I have been researching this issue through various forums I belong to and I seems the problem may be the valve guide/seals. Can anyone give me an idea what it will cost to have this work done?