I've owned two W123 diesels, an '85 300D and my current car, and '82 300CD. Their engines hold 8 quarts of oil. And yet, when I drain the oil preparatory to changing it, I only get about six quarts out. (I put the used oil into 1-gallon plastic milk jugs, so this volume isn't difficult to estimate.)
I understad that some amount of oil is going to cling to the inside of the engine as a film -- but *two quarts?* Fully twenty-five percent of the rated capacity? There has to be some cavity or other in the engine where a substantial volume of dirty oil is pooling, but damned if I can think of where it'd be. Yes, I open the filter canister when I drain the used oil, but that's only good for a pint or so.
Geoff