A detergent additive package, fuel additive concentrate, fuel composition and method for operating an engine on an unleaded gasoline fuel. The additive package includes a Mannich base detergent mixture, wherein the mixture contains a first Mannich base detergent component derived from a di- or polyamine and a second Mannich base detergent component derived from a monoamine. A weight ratio of the first Mannich base detergent to the second Mannich base detergent in the mixture ranges from about 1:6 to about 3:1.