There are additives in a bottle at the parts store that are sold specifically for this purpose. It'll do absolutely no harm to try some. I doubt of pouring Marval Mystery Oil in and letting it sit will do anything. It isn't a solvent.
But I gotta tell ya, at 24 years and 518,500 miles, it may be futile. In addition to wear on the cylinder walls and rings, rings lose their tension. When they're originally installed, they have spring tension against the cylinder walls. After being heated up and pouned on million and millions of times, they lose this tension. They become a squeegee without pressure.
I suspect the problem is not coked rings. I suspect the problem is tired old rings, beaten down with age, in worn out cylinders. But try the additive anyway. You've nothing to lose.
Congratulations, by the way. That mileage is one heck of an achievement.