In my experience with 4 MBZ diesels, all of them exhibited SOME oil on the top of the valve cover. Gas cars probably do it just as much but the oil is so much cleaner that you don't notice it.
Obviously, this comment does not justify the presence of
more than perhaps a teaspoon of oil per week. If you have more oil than that, something is wrong and it should be fixed.
'85 300 SD, 165K-Sold
'88 300 TE, 165K-Sold
'64 Porsche 356C Cabriolet
'86 560SL 124K Miles
'94 320E Wagon, 74K Miles