MB engines are pretty well known for using oil if overfilled, and synthetic oils are good about staying up in the engine instead of draining back into the pan, with the result that if you are in the habit of checking the oil while filling up, and add to keep the oil at the top mark, you may be "overfeeding" the engine, and it will cheerfully burn off the excess.
This has been verified by my mechanic friend, who actually bought a customer's car once that had "an oil consumption problem" -- problem vansished when he didn't add oil until the bottom mark -- oil level dropped to halfway between the marks and then stayted there for 3000 miles.
However, if you do have real oil consumption, you may have a ring problem. Dirty rings will stick and cause excessive oil consumption when run around town, but after acouple hours on the highway, unstick for a while and oil consumption drops to nothing. The 220D does this -- four quarts a tank, until you get it on the highway for about four hours -- no oil usage to speak of then until it gets cold again.
What I would do would be to check the oil only first thing in the morning, engine cold. Monitor lever daily, and only add a single quart if the level reaches the bottom mark on the dipstick. Your problem may go away!
If not, it's time for a leakdown test and plug examination.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!