Disulfide oil (DSO) compounds recovered as a by-product of the mercaptan oxidation of a hydrocarbon refinery feedstock and their oxidized derivatives, oxidized disulfide oils (ODSO) are effective as a diluent to lower the viscosity and thereby improve the pipeline transportation properties of heavy oils, and particularly of wellhead crude oil. The use of the DSO and/or ODSO compounds as diluents converts an otherwise extremely low value or waste oil product into a valuable commodity that has utility in improving the transportation properties of heavy oils, particularly in oil field pipeline applications.