Multigrade engine oils of high viscosity index (VI) are required for modern internal-combustion engines. Such a high VI is difficult to achieve by the conventional processing of petroleum fractions. Therefore, VI improvers are added in the mineral base oils to achieve the desired VI. The enhancement of VI of mineral base oils (100 N, 150 N and 500 N) by the addition of VI improvers [polyisobutylene(PIB), polybutadiene rubber (PBR)] has been studied. VI of blended oils, made from the mineral base oils 100 N, 150 N and 500 N by the addition of PIB and PBR, is found to pass through maximum. It was observed that the occurrence of maximum depends on the mineral base oil used and the type and concentration of VI improver.