The present invention is directed to a method for improving the performance of a sooted diesel oil and controlling soot induced viscosity increase by adding to a major amount of a diesel oil a minor amount of a composition comprising at least one compound having the formula Mo.sub.3 S.sub.k L.sub.n Q.sub.z and mixtures thereof wherein the L are independently selected ligands having organo groups with a sufficient number of carbon atoms to render the compound soluble or dispersible in the oil, n is from 1 to 4, k varies from 4 through 10, Q is selected from the group of neutral electron donating compounds such as water, amines, alcohols, phosphines, and ethers, and z ranges from 0 to 5 and includes nonstoichiometric values.