While Tank is likely right on the mark, I would atleast check a few things before throwing in the towel.
First, if you are burning a quart of oil in under 100 miles, you must be trailing a cloud of blue oil smoke, or even white stuff. Have you noticed this? If not, that oil is probably not going through the combustion chamber and out the exhaust.
If you notice a cloud, but mostly only at idle, you should check the vacuum pump discharge line (I assume your 300D has one) that goes to the intake manifold. This is a translucent white line from the front, driver's side of the engine that crosses over in front of the engine and goes to the intake manifold, or the air cleaner on some models I believe. If this line is black you have ruptured the pump diaphragm. Not a lot to have it replaced, but chances are you should have the unit rebuilt, which will at least double the cost. Good investment though if the diaphragm is ruptured.
I cannot think of any other obvious things to check at the moment, but that is a lot of oil and it should be showing as leakage or smoke, neither of which were noted as part of the problem in your original post.
So, good luck and I hope this helps. Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)