Swarfega is probably a good idea, radiator flush is formulated more to get rid of scale and probably not at all effective on oil. Look for a shop with a proper flush machine that uses compressed air and forces bubbles through along with hot water. Skimming as much as possible off the top, as suggested, before hand and maybe even after. Take your video footage to the police and/or post it. The used car market seems to be full of these assholes lately and it's just getting worse.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)