Viscosity index is one of the most important parameter for the lubricating oils performance. The effect of additives on the viscosity index for mineral lubricating oils SN500 and SN150 have been studied by adding castor oil, olive oil, corn oil, sunflower oil in addition to Glue Adhesive and poly isotridecanol ethoxylated used by North Oil Refineries Company for comparison the results. The viscosity has been measured at 40°C and 100°C according to ASTM D445 and viscosity index calculated according to ASTM D2270. It has been found that corn oil has the best effect of the vegetable oils in improving the viscosity index of the mineral base oils. The influence of additives on the pour point was also evaluated, castor oil showed the best result of all vegetable oil in depressing the pour point of the Base oil.