I would always do a double change on a second hand bike. For a bike in the condition you described, I would keep changing the oil every few hundred miles until it came out clean, and only then, put the good stuff in. It is not really the distance between changes that matters so much as the heat cycles. Get it up to temperature a few times so the fresh oil can melt and dissolve the built up gunk.
All sounds like a load of bollocks to me. My understanding is that car oils have molybdenum in them, this is actually a slip agent, it helps reduce wear on car engine moving parts by effectively adding extra slip, great, but not on a multi plate wet clutch.
Had to fit new clutch, flush the engine twice to get rid of the car oil to a Bandit 1200 previous owner had used gave a new meaning to slipping the clutch expensive oil and filter change. I was always told never use car oil in a wet clutch bike.
I had read somewhere about not changing over from semi to fully if a motor has been run on one type for a long time but tbh I don't really buy into that and I reckon i'll go with some fully synth in both vehicles now.
Another of the many oil myths on the internet, along with don't use synthetic until the bike has done 10k miles etc etc.
Haven't had a go in a good oil thread for years. This is just like old times!
As you rightly say, if an oil meets Yamaha's specs and stays 'in grade' for whatever oil change interval you work to, then it doesn't matter what pictures are on the can.