This invention relates to fuel oil, especially middle distillate fuel oil, compositions comprising middle distillate fuel oil and incorporated therein an incorporated therein an additive composition comprising (a) at least one fuel-soluble or fuel-dispersible neutral alkaline earth metal compound and/or at least one fuel-soluble or fuel-dispersible neutral alkali metal compound, and (b) at least one fuel-soluble or fuel-dispersible transition metal compound, characterized in that the fuel oil composition contains at most 0.05 mass % of sulfur, the total metal content derived from (a) and (b) in the fuel oil composition is at most 50 ppm by mass, and the mass proportion of (a) to (b), based on metal content, is in the range of from 1:99 to 99:1.