According to the manual for my '96 C280, it takes exactly 7.0litres or 7.4 US Quarts. This is correct if you do oil and flter (I put in exactly 7 litres when I do mine and it takes it right to the full line). I would talk to the service manager and ask him how the oil they add is measured. It's a remote possibility also that they do not change the filter each time, but add oil as if they did.