Sulfides of the sym-triazine series are synthesized using, as key compounds, the fraction of C-2-C-5 mercaptans isolated by processing wide-cut hydrocarbon fractions of Orenburg gas condensate. Compositions of the industrial antioxidant OMI with the sulfides synthesized were shown to have high performance as additives enhancing the thermooxidative stability of straight-run diesel fuel blended with 20% light catalytically cracked gas oil.