Marvel Mystery Oil. Cleans and lubricates fuel injectors and carburetors. Improves gasoline mileage. Reduces and prevents varnish and gum build-up. Extends spark plug life. Safe for catalytic converters and oxygen sensors. - ADDED TO ENGINE OIL: Prevents valve sticking and clatter. Fortifies properties of engine oil, prevents breakdown caused by extreme temperatures. Promotes easier cold weather starts. Reduces and prevents acid and sludge formation. - FOR ALL MARINE ENGINES: Inboard and outboard.