I believe I remember the Tappet brothers arguing that you can burn a quart of oil every five hundred miles without it being visible in the exhaust.
I have found that using different brands of oil and different viscosities can have a big effect on how much oil the engine consumes. For instance, using 10-40 in my Olds 455 in the summer when temperatures are over 105 will result in significant oil usage. Put in 20-50 and the oil usage drops precipitously.
I think if you look at the Mercedes literature you will find that using that much oil is well within acceptable limits for their diesels. I would not worry about it if it were my car.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13