I'd buy your used motorcycle.Why not synthetic changed at the Mfg interval?
IMO, if you told me you use synthetic and go 2X the mfg interval miles between changes, and your neighbor is selling the exact same motorcycle but used SuperTech Cheapo oil, but changed his at the mfg interval, I'd buy your neighbors motorcycle.
Have you ever looked at a used car/motorcycle and the seller has fresh oil in it, but has no idea when it should be changed next? Doesn't give you a warm fuzzy that it's been done other than have new oil in it to sell it.