Last owner apparently not a big fan of changing the oil

If you were to run the engine with the cover off, you would see how the oil flows in there and how the high spots and lower areas catch only little splash droplets that just sit there and age. A flush product wont get to the high spots to wash it. The more you disturb that stuff the more will fall in and go to your oil pick up screen. Might be best to just close it up. Or pull the oil pan and clean it all including the pickup screen. Me, I think I would wait until it needs a timing chain.
And about a fuel flush i.e. diesel or kerosene, not on a high millage engine. That'll flush out a well worn bearings. The bottom end will come apart. Seen this too many times where a fuel pump failed and filled the crankcase with gasoline.