To obtain a fuel oil additive highly able to disperse sludge by mixing an ester, amine salt or amide of an olefinic hydrocarbon/maleic anhydride copolymer, an amphoteric surfactant and an organic acid.

The olefin hydrocarbon used is usually a 2-20 C olefin. The hydrocarbon is copolymerized with maleic anhydride in a molar ratio of generally 0.4:1.5-1.5:1.4 Average mol.wt. of the copolymer is usually 2,000-6,000. This copolymer is converted into an ester, an amine salt or an amide by a usual method. The amphoteric surfactant is particularly desirably a carboxylate type or a phosphate type one. The organic acid is particularly desirably a 10-18 C unsaturated fatty acid. The mixing ratio among the respective components is usually such that the ratio of the copolymer to the surfactant is 25:75 to 75:25 by weight and that the ratio of the total weight of the copolymer and the surfactant to the organic acid is 10:90 to 90:10.