Is it the M102 2.3L 8-V engine or the M111 DOHC 16-valve engine?
The 102 is not a big oil burner, but a litre per 1500km's isn't bad. For the later model M111 DOHC engine, that's probably quite a bit of consumption.
In the future, the model year and more details of the car are helpful...
1998 C230 "Black Betty"