If you feed the engine cleaner into the intake manifold, it will never get into the valve cover or settle into the oil pan unless you have another problem. It will travel into the intake manifold, cleaning it, and then into the combustion chamber passing by the valves, cleaning them, and out of the exhaust. If you don't feel comfortable with it, you can always buy some good ole BG to put in your gas tank and it will work its way through the fuel injectors and eventually into the combustion chamber. It won't do much for the intake manifold however.