I think Lubrizol website may have given us a hint.
They state that high detergent levels which would help with engine cleanliness work against fuel ecomonomy and the friction modifiers in the oil. The Dispersants also work against fuel economy.
They state that high detergent levels which would help with engine cleanliness work against fuel ecomonomy and the friction modifiers in the oil. The Dispersants also work against fuel economy.