The back side of intake valves can get heavy deposits. This affects smooth intake flow. Red Line Complete Fuel System Cleaner cleans injectors and valves using detergents and no solvents. Safe on pintle-less injectors. I chose to use a small amount with every tankfull as per directions for the last 5000 miles and mileage has increased 3MPG. My engine(NA) is really smooth. If in your area fuel is heavily loaded with oxygenates, it will leave much more deposits that will affect mileage.