The first time I got my valves adjusted the mechanic spent ten minutes talking me OUT of buying a new valve cover gasket. While I dont have a lot of experience with Mercedes diesel engines, I have always replaced a valve cover gasket if the cover needs to be removed.
Sure enough, after getting home there was oil all over the driveway, needless to say that was the last time I went to that mechanic (who is supposedly a reputable Mercedes guy).
As I said, I'm far from completely knowledgeable about these cars, but in my opinion the piece of mind is well worth the price of a valve cover gasket.

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1985 300SD
216,000 miles