A method of stabilizing asphaltenes in a petroleum hydrocarbon fluid comprises contacting the petroleum hydrocarbon fluid with a mixed phenol aldehyde derived from an aldehyde and two or more unsubstituted or alkyl-substituted monophenols, the two or more unsubstituted or alkyl-substituted monophenols comprising a first phenol which is a C1-20 linear or branched alkyl monophenol; and a second phenol which is unsubstituted or a C1-20 linear or branched alkyl monophenol different from the first phenol, wherein the molar ratio of the first phenol relative to the second phenol is about 4:1 to about 1:4.