The invention relates to a method for formulation of a synthetic gas oil or an additive for gas oil in which an alkyl-aromatic compound or a mixture of alkyl-aromatic compounds is selected based on at least one parameter that is selected from the group that consists of the number of cycles of the aromatic core, the number of alkyl chains that are grafted to the aromatic cycle, the length of the alkyl chain or chains, the position of the aromatic cycle or cycles on the alkyl chain or chains of said alkyl-aromatic compound or compounds such that the cetane number of the synthetic gas oil or the additive for gas oil is greater than 30. The invention also relates to a process for the production of alkyl-aromatic compounds for use as a gas oil or additive.