[Flip]

I've heard of a few various concoctions being used to try and free off stuck piston rings without stripping the motor but I don't have any experience of them personally.

Everything from proprietary lubricants such as Red-X as Mr G mentions to pouring Diesel in through the plug hole and leaving it a few days with the theory being that the moment it doesn't disappear then the rings are freed and the seal has been re-made.

Obviously anything that get past the rings and into the crankcase would need to be drained out before starting and it would be wise to give the motor a couple of oil changes before feeling settled it was back to normal.

As for 'mayonnaising'- mine never suffered too badly from it with the factory set-up but since removing the 'warm air' return pipe from the catch tank to the air box it never does when it has a run out now whatever the ambient temperature is.