We have had good luck freeing stuck oil rings using GM's (AC Delco) product X-66p. This is a pop-top can. For real effect we have poured some into the cylinders (through the spark plug hole) and hand cranked the motor through a couple revolutions. This is some nasty stuff don't breath it or get it on your hands. Also don't leave it over a few hours sitting on aluminum.
We usually let it sit for a half hour after cranking by hand. Spin it once by the starter with the ignition disabled to be sure there isn't enough in the cylinder to cause damage (place rags over the holes to keep the stuff out of the air). Do all this cranking with the spark plugs out. Also it should be done to as hot a motor as you can stand working on.
After this we suck the rest of the can through a plenum port (center of intake)into a running motor allowing enough to pass that the idle is affected. After using up the can stop the motor and let it sit for twenty minutes then start it back up in a well ventilated area, where you won't alarm anyone. When you start it back up all that carbon is going to burn and make the area look like a brush fire.
The instructions for everything but the first hand flushing are on the can.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician