Waste lubricating oils may be re-refined with organic solvents that dissolve base oil and segregate the additives and solid particles. The present paper emphasizes the composition effect on the efficiency of a ternary solvent composed by n-hexane/2-propanol/1-butanol. Using the ternary diagram of waste oil/basic organic component/polar compound containing 3 g/L KOH where the phase envelope and the curves of constant sludge removal are plotted, it is proposed the composition of 0.25 waste oil/0.35 n-hexane/0.40 polar compound (80% 2-propanol + 20% 1-butanol, with 3 g/L KOH) for the process. A contribution for the understanding of polymethacrylate flocculation supported on infrared spectra is given.