For the most part, this is true. Rings do a pretty good job of sealing combustion pressure, but it IS still a metal on metal seal. There will always be a certain amount of leakage that gets by (more so on older, more tired engines). I work in the auto repair industry for a living, and one way (only to illustrate a point) we use to help aid in determining ring problems is, after running a compression test and you find a cylinder with low compression say, and you need to determine where that compression is leaking to, right? So we squirt a few squirts of engine oil into the combustion chamber through the spark plug hole, and then crank the engine over a few revolutions to slosh that oil around the cylinder. Then we redo our compression test, and if the compression suddenly returns to almost normal or normal levels, then we know that we have ring problems (either just worn, broken, or whatever). If the compression stays low after doing this, then we do a leakdown test to hunt for bad valves or whatever else. But the point is, that oil always improves the seal. I seen engines that got flooded real bad that wouldn't even start because the gas rinsed all the oil off the cylinders and eng lost all it's compression. So when you merely cranked on the engine, it would never start, but if pull-started (eng. had enough speed to slosh oil around on the cylinders), then the engine would build compression and start. This is a simple law of physic, but the oil is actually the seal between the two metals (of the rings and cylinders). Just food for thought
