[James Brown]

crud, carbon, gunky overheated oil, get collected in the piston lands and trap the piston rings mostly in the compressed state allowing gas to leak past the rings reducing performance and compression. Some additives or "motor in a can" use strong solvents to try to melt that crud out from the inside. Guess what that stuff is doing to the seals and small leaks that have plugged themselves up over the years. Of course the best thing is disassembling and cleaning, using good oil, checking wear specs, etc. Most miracle fixes don't.