Myself I would use Kroil. Have heard of all sorts of stuff used though from engine oil to diesel or gasoline.
Let sit for while LEAVE SPARK PLUGS OUT or put em back in LOOSE then remove before turning.
Then get wrench and attempt to turn GENTLY first one way then the other. I say gently since you do not REALLY know what the problem is. If just stuck rings you might be OK to get it to rotate. Think I would be tempted to take the head off and have a look see.
Should also make double sure transmission is not jammed.
Look for obstructions to ensure flywheel is not jammed.
Dan Bentler
