A method for refining used motor oil using two or more hydrotreating reactors arranged in series. The used motor oil may be vacuum distilled to produce an unrefined gasoil. The unrefined gasoil may then be hydrotreated in a first hydrotreating reactor with hydrogen operated at a temperature ranging from approximately 245° C. to approximately 260° C. to produce a hydrotreated gasoil. The hydrogen may comprise a mixture of fresh hydrogen and recycled hydrogen recovered from the last of the two or more hydrotreating reactors. The hydrotreated gasoil may then be hydrotreated in one or more additional hydrotreating reactors operated at temperatures ranging from approximately 260° C. to approximately 330° C. to produce a refined gasoil. The first hydrotreating reactor may remove a substantial portion of metallic impurities from the unrefined impurities, while the one or more additional hydrotreating reactors remove a substantial portion of heteroatom impurities from the unrefined gasoil.