Thank you for your reply. Last year the when the dealer mentioned that it had a sensor to tell you when to change the oil, we had them put in synthetic for $110 I think, so it has that in it now. I guess I would just assume keep using synthetic, but just count the miles and change it maybe every 7500 miles. The car has 175,000 miles on it now but runs and looks like new.

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1996 E320
2000 C230 Kompressor
1988 190e 2.3 - 225K miles, owned for 7 years. I cannot say enough good things about this car. Very well built, even at 225 it ran like new.