The invention provides a fuel oil composition comprising a major proportion of liquid hydrocarbon middle distillate fuel oil having a sulphur concentration of at most 0.05% by weight, and a minor proportion of a dispersant additive obtained by reacting, in a molar ratio A:B in the range 4:3 to 1:10, (A) a polyalkenyl derivative of monoethylenically unsaturated C4-C10 dicarboxylic acid material in which the number average molecular weight (Mn) of the polyalkenyl chain is in the range from 850 to 1150 with (B) a polyamine of general formula H2N(CH2)m—[NH(CH2)m]n—NH2  (1) where m is in the range from 2 to 4 and n is in the range from 1 to 6; a process for the preparation of such a fuel composition, and a method of operation a compression-ignition engine with enhance injector cleanliness using such a fuel oil composition.