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I've never seen anything like that (with lots of Volvos past and present). But, being soot-like, I think one possibility is the stuff that would clog the air/oil separator box (that leads to your namesake flame trap) -- perhaps it built up and then (luckily for you) fell back into the sump?
I use synthetic oil (and have since 1975), and it seems (because others have also experienced the same thing) to prevent build-up and clogging of the flame trap, oil separator and related hoses. I recommend it.
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