By the proper selection of components and compositions, a composite solvent to recover base oil from used lubricant oil has been formulated. The composite solvent has two single components: methyl ethyl ketone (MEK) and 2-propanol. The best extraction results were obtained when the single solvents selected were mixed at a 2-propanol/MEK ratio of 3 g/g. However, this solvent was still unable to completely remove metals and oxidation products. To solve this problem, very small quantities of KOH (from 1 to 7 g/(kg of solvent)) were added to the composite solvent and its effect on both the extraction yields and the quality of the oil recovered was determined. A KOH concentration of 2 g/(kg of solvent) was found to be the most appropriate. The vacuum distilled oil pretreated with this solvent (2-propanol/MEK at a weight ratio of 3 g/g with 2 (g of KOH)/(kg of solvent)) was almost similar to a SN-130 virgin oil and, therefore, suitable for the formulation of new lubricants.