I had a similar situation with my 560sel and it needed the proverbial valve job....top engine overhaul. If yours hasn't had an overhaul in its history, then it's probably past due...especially if you find yourself always having to add oil.
2002 ML320 (thereby making my 560SEL my "spare).
1992 500E, pearl black, 27,000 original miles as of 7/2003.
1990 560 SEL, black pearl metallic, sweetness to drive. Fully outfitted with driving countermeasures (Beltronics radar/laser detector and Uniden BCT-12 Bear Tracker scanner).
1985 Porsche 911 meteor metalic gray, a rocket to drive.
1988 Chevy Silverado pick-up (gotta have something to drive to the rodeo!)
1978 Corvette Indy pace car for show
1971 Opel GT for sentimental reasons