I owned a 1992 300E a few years ago. (88k on it) I believe MB says a quart of oil consumed every 1,500 miles is actually acceptable (I think that's the mileage number-I can't remember exactly though). I was burning a quart every 1,000 to 1,500 miles though. With most other cars this would be unacceptable. A lot of mechanics thought that was normal. One recommended I replace the valve guides anyway. Sure enough they were all brittle. (I think that was the name of them-this was 6 years ago). The valve guides are cheap maybe $30 for the parts, but the labor is time consuming. I did it myself and it took about three-hours. Check into that @ a MB dealer, as far as what the oil consumption rate should be.