A stabilized nano-Fe6-iron-crown ether complex is added to a heavy oil to reduce the viscosity of the heavy oil. The complex may be formed by preparing a solution of an iron salt and an oligomer crown compound in dialkylamine or diethylamine. Sodium tetrahydroboron (NaBH4) and dialkylamine or ethylendiamine are added at a temperature of 0-10° C. The mixture is heated to room temperature and boiled, thereby converting the formed iron (II)-borhydride complex (Fe(BH4)2) to a crown ether—iron-hydride complex [CWFe06].(2H)6. At higher temperature this last complex is converted to the Fe06-crown nanocomposite complex. The prepared nanoparticle Fe6° complex may be stabilized by inclusion into a cavity of a macrocyclic compound, and may be (1) added to diesel fuel to reduce NOx emissions upon combustion of that fuel; (2) added to lubricating oils as an anticorrosion additive; and (3) used as an additive to secondary recovery processes within liquid hydrocarbon formations to increase the sweep efficiency and recovery factor of water-flooding operations.