A method of manufacturing high-quality naphthenic base oils comprising a high aromatic content and a large amount of impurities with a boiling point higher than that of gasoline. High-quality naphthenic base oil may be manufactured from light cycle oil (LCO) and slurry oil (SLO), which are inexpensive, and have a high aromatic content, a large amount of impurities, and which are effluents of a fluidized catalytic cracking (FCC) unit. The method also relates to the pretreatment process of a feedstock, where the amounts of impurities (sulfur, nitrogen, polynuclear aromatic compounds and various metals components) in the feedstock are reduced.