The various engine-cleaning additives are kerosene. It dissolves bad stuff, loosens stuck rings etc. But I did a miracle clean of an old Skoda by replacing the oil with Turbinol. a very thin, high-detergency lubricant used in big turbines. The car had used a lot of oil and smoked heaps. I ran it for 15 minutes on the Turbinol. dumped that and filled with more Turbinol, drove it over a little mountain range, dumped that and filled with Castrol. No more smoke, better compression, all good.