A similar thing happened to me last fall on a 300sd. I bought it one afternoon, drove it home, adjusted the valves, and drove it to work the next morning. In 12 miles I managed to loose 6 or 7 quarts - still trying to figger out where it went.
I shut the car off and started coasting when i saw the oil gauge fluttering between 2 and 3. it was too late. The car still runs, but it has some knocking, and smokes a bunch.
What bugs me is that I still don't know where all that oil went in 12 miles. I suspect that the car had bad blowby, and I lost lots out the vavle cover gasket, maybe it was not sealed tight - I did have to wrestle it into place, and I was in a hurry. Still, the engine bay did not look like it had 6 quarts scattered around. Some must have gone out the tail pipe.
I drove the car about 50 miles before I did the valve adjust - no oil loss, but something happened when I took off that valve cover.
The upshot is that I will be parting out an '81 sd soon. And I still want to know where all that oil is.

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andy t
'78 300d
'95 volvo 850, wagon
'86 300sdl - engine out, maybe I'll have it rolling by June

whole bunch o' bicycles