I just purchased a 1998 Z3 from a local dealer (not BMW)and I was wondering when I should change the oil. The car has 13,000 miles. I was told that the car will "tell" me when to change it (the little green bars - of which I have three left). Or does that inform me of an oil service which entails more than just a change. The last thing I want to do is make an improper assumption about oil changes. Thanks in advance.