Therein lies the rub. Modern oils are marketed at "keeping pistons cleaner than xx standard", or "Superior protection against engine sludge", or "When you need to protect and preserve a steadfast engine" NOT at cleaning engines. It's an old wives' tale. Varnish, sludge, and coked oil are extremely tough substances to remove, and it's just not financially feasible for oil manufacturers to provide something that has the muscle to clean for $15/gal.
Cleaning sludge and clearing piston rings is driven by polarity of oil molecules. Lower group oils (GRP I and II) have much higher polarity and will have a better shot at cleaning, but they are still extremely low on the scale. Search JAG's post on Valvoline Premium Blue Restore oil ($70+/gal), it is formulated from an extremely polar base oil which is designed to clean piston rings and sludge. Other than that, your only other real choice is to use a solvent-based cleaner.