The lubricating oil is of great interest as it provides a thin film on moving parts to reduce the friction, heat, wear between them. Recently, with the rapid increase in quantities produced of the used lubricating oil, its disposal has become a global environmental issue due to its impact on all life aspects. The aim of this work is the refining of used lubricating oil so it can be used again. This was investigated using the acid-clay process. The most important step in this work is the desludging reaction. The Response Surface Methodology is used to find out the optimum conditions for the desludging reaction, and the selected parameters were time, temperature, and acid to oil ratio. The temperature ranged from 40℃ to 60℃, the stoichiometric ratio of acid to waste oil ranged from 10% to 20%, and the reaction time ranged from 60 to 120 minutes. The design is applied and the predicted optimum conditions are the temperature of 40℃, 20% acid to oil ratio, and reaction time of 60 minutes. To predict the results, five runs were performed at the optimum conditions to predicate the standard deviation of 0.739 and the human error which did not exceed 5.7%. The characterization results of the product of the refining at the flash point of 180℃, carbon residue of 0.487, kinematic viscosity at 100℃ of 17cSt, the water content of 0.01 %, ash content of 0.02 %, and viscosity index of 110.2996 were found in complies with the Egyptian standards.