In another embodiment of the present invention, the additive mixture can be prepared by reacting a carboxylic acid or reactive derivative thereof such as an ester, amide, acid halide, anhydride or ketene thereof with at least l.ln moles of an amine per mole of carboxylic acid R[COOH] n or reactive derivative thereof where n is equal to the number of carboxy groups in the carboxylic acid.