crit1, I can't believe a MB dealer's Service Department is telling you not to use Mobil 1 oil and the rest of the diagnosis. Are you actually taking your C280 to a MB dealer? If so, insist that they replace the head gasket again on their nickel and correct the other issues that they overlooked the last time you paid to have the head gasket replaced. If not, either go to a MB dealer or a reputable MB-trained independent shop and get the problem diagnosed accurately and repaired properly.

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Fred Hoelzle