I had the exact same thoughts. I drained the oil, filled the crankcase with mineral spirits and shook it around, dumped that, repeated that, and then put some fresh oil in it. I ran that or a few minutes at full throttle, and then drained that. The oil was grayish, and seemed thin, like there was definitely mineral spirits left in it, which suprised me since I had dumped it out pretty thoroughly. I ran out of oil, so left off there for the day. Tomorrow, I will add some fresh oil and see what happens.
What I'm wondering, from those who have one, is could it be just general bore wear on an aluminum block that doesn't have a cast iron sleeve?
I'm also having trouble thinking about what could be causing it. It runs good, doesn't smoke, and has been well maintained. Cranks are too hard to shed material, but perhaps the plain bearing?