Around twenty years ago I was running a mom and pop auto parts store. A fellow came in and bought a quart of engine flush. He looked like the type that would get really p.o. if I warned him about not using it so I kept my mouth shut. The subject car was a early seventies Olds with a 455. It sounded a little ragged, but nothing too alarming. The next day the same guy came flying in the store wanting to kill somebody because he dumped the flush in and drove around for a couple of hours.....until a rod came through the block.

Engine cleaner is kerosene, and shouldn't be used for the same reason you shouldn't use detergent oil in a car that had been run on non-detergent oil. All the crud gets knocked loose and plugs up the motor(esp the oil pump pickup), ergo no oil pressure... and a thrown rod.

Frequent oil changes are best, though I have been guilty of putting a little Sea Foam in the oil to free up lifters and rings. I always worry about reducing the lubricity of the oil. Anyways, just my .02 cents.