The effect of Catechin-containing natural antioxidants, on the stability of lubricating oil has been assessed and evaluated. The base and the formulated oils samples were both subjected to the same reaction conditions designed to simulate a working crankcase engine environment. The kinematic viscosity, pH and the total acid number (TAN) were determined to assess the effectiveness of catechins (from groundnutshell extract) as an oil additive. The values of the kinematic viscosity for the formulated oil were much higher than the base oil, while the total acid number content for the base oil was found to be higher. Both the base oil and the formulated oil, showed similar pattern of deterioration as temperature of the reaction increased. However, the formulated oil proved to be more stable than the base oil due to the presence of the antioxidant.