Replace the valve stem seals. I had the dealer do it for about $390 1.5 years ago on my 300E. Went from burning a quart every 400 miles to using less than 1/3 of a quart per 5000!

Spark plugs are perfect and clean on mine after 15k miles. They used to foul after only a few hundred.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)