An additive composition comprises a polymer (A) and a condensation product (B). Polymer (A) comprises the following monomer components: (i) one or more compounds of formula (I) wherein R1 is hydrogen or CH3; and R2 is a hydrocarbon group having 6 to 30 carbon atoms and is a straight-chain or branched-chain alkyl group, or an aliphatic or aromatic cyclic group; (ii) one or more compounds of formula (II) wherein R1 has the meaning above and wherein R3 is hydrogen or C1-C22 alkyl; each R4 is independently hydrogen or C1-C22 alkyl; R5 is hydrogen, a substituted or unsubstituted aliphatic or aromatic cyclic group, or a substituted or unsubstituted straight-chain or branched-chain alkyl group having 1 to 22 carbon atoms; n=0 or an integer from 1 to 22; and m is an integer from 1 to 30; and (iii) one or more compounds of formula (III) wherein R6, R7, R8, R9 and R10 are each independently hydrogen, a straight-chain or branched-chain alkyl group having 1 to 20 carbon atoms which may be substituted or unsubstituted, hydroxyl, NH2, or wherein two or more of R6, R7, R8, R9 and R10 may together form an aliphatic or aromatic ring system, which ring system may be substituted or unsubstituted. Condensation product (B) comprises the product formed by the reaction of an aliphatic aldehyde or ketone, or a reactive equivalent, with a substituted phenol or mixture of substituted phenols. The weight:weight ratio of the polymer (A) to the condensation product (B) is from 1:20 to 20:1