The present invention relates to alkylphenol-aldehyde resins containing oligo- or polymers having a repeat structural unit of the formula where R1 is a branched alkyl or alkenyl radical which has from 10 to 40 carbon atoms and bears at least one carboxyl, carboxylate and/or ester group, R2 is hydrogen or R1 R3 is hydrogen or an alkoxy group of the formula -(A-O)m—H, A is a C2- to C4-alkylene group, m is from 1 to 100, R4 is hydrogen, a C1- to C11-alkyl radicals or a carboxyl group, R5 is C1-C200-alkyl or -alkenyl, O—R6 or O—C(O)—R6, R6 is C1-C200-alkyl or -alkenyl, n is from 1 to 100 and k is 1 or 2.