I have had the vacuum pump diaphragm fail and start to pump oil through the engine at an alarming rate (and 250miles to a quart is getting near the point of alarm) but it was readily apparent that something was amiss. When I stopped the car would be enveloped in a thick cloud of white smoke, and it made some extra knocking noises.

From what you say, there is a genuine mystery. I get 900 to 1500 miles to a quart in my 240D and a good bit if that leaks past the rear seal. But the smoke is noticable at idle and under heavy loads, like going up hill at highway speeds. To be unable to find evidence of nearly 6 times that much oil is unusual.

Has the car always used oil at that rate, or has it suddenly started or did it get to this point slowly? It sure seems like something is letting a significant quantity of oil go somewhere it is not intended and after a few quarts there should be some visible evidence.

I remember once adding oil to the injection pump on my 1975 240D, and being unaware of the overflow tube. fter adding two quarts I found my shoes covered in oil and my buddy's driveway soaked in it (it was Mobil 1 5W30 back in the old days and it was an expensive lesson). I was wondering where all this oil could go, but once again, when the oil went where is was not intended to be, it let me know about it pretty dramatically.

Jim

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Own:
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

Owned:
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)