thanks rscurtis, that's helpful.

vstech, i didn't pull the pan - working on the street here - but siphoned 40ml of oil out of the bottom of it and let that flow over a white paper towel. there was no debris in it at all, not even the faint trace i saw in the filter.

more and more i'm thinking that one piston ring may have broken off. that's just a hypothesis. thanks everyone for chipping in with info. what to do next...

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'77 240D, 504H, OM617.952, etc.