Abstract: An oil desulfurization method may be used to desulfurize various oils, such as used motor oil, crude oil, diesel, high sulfur fuel oil, mid sulfur fuel oil, off-spec fuel oil, and off-spec diesel, to produce a finished product of lower sulfur oil and a high sulfur fuel oil or sulfur containing oil product. Preferably, the method may include the steps of: mixing an oxidizing material with sulfur containing oil to produce a first mixture; subjecting the first mixture to at least one of heat and pressure to oxidize the sulfur in the first mixture; mixing at least one solvent with the first mixture to produce a second mixture; and separating the second mixture to produce a low sulfur oil product and a third mixture, the third mixture having a high sulfur oxidized oil and the at least one solvent.