Use in a gas oil fuel composition, which preferably comprises a Fischer-Tropsch derived fuel, of a compound according to formula (I): wherein: R1 to R5 are each independently hydrogen or a C1-10 alkyl group, where such alkyl groups may be the same as or different from one another; X is a nitrogen- or oxygen-containing group, for the purpose of reducing the cetane number of said fuel composition; preparation of such a fuel composition; and operating a fuel consuming system.