Seafoam, on the other hand, is more of a cleaner. People dump it in the crankcase to break up sludge, or into the intake to de-gunk valves and chambers. If you've inherited a neglected engine or ...

Used right, in the right engine, they can help. Used wrong, they're a waste of money or worse. If your engine is healthy and your oil is fresh, save the $15 for something useful. Like gas. Or tacos.