There's two distinct varieties of the 104 motor - the early 3.0 liters with CIS, and the later 3.2 and 2.8s with HFM engine mgmt. The bottom ends are fine on both but the top ends on the 3.0 liters - 1990 to 92 on some cars, 93 on others - are generally considered not to be as strong as the either the earlier 103 motors or the later 104s. Mercedes had fallen behind BMW and the Japanese on 4 valve technology and there is a sense that they rushed these cars to market. It was there first in-house mass market 4 valve/cylinder cars. They're just not as tight - they have a tendency to burn oil and not to have the top end life of the other sixes. I had a 90 SL that needed a valve job at 40k and it was my understanding that there were hundreds more like that. I don't think that means your car is doomed or anything - I have a friend with 130k on a 3.0 liter 104 with no serious problems.