Nowdays, production of high quality engine oils is one of the worries of our country’s engine oil industry. In this study, the behavior of four commercial polymers used in nation motor oil industry in obtaining the best viscosity index improver was investigated. In this regard, viscometric behavior, shear stability and polymers’ performance for engine cranking were compared at low temperature. In this respect, viscometry behavior of polymer solutions with different concentrations (0.3, 0.5, 0.7 and 1% by weight) in terms of physical chemistry parameters such as specifc viscosity, intrinsic viscosity, viscosity ratio, thickening power and viscosity index were compared with each other. The activation energy of each polymer solution was calculated using the Arrhenius equation and polymer performance was evaluated with respect to activation energy changes and lower oil temperature sensitivity.
In continuation, the shear stability of these commercial polymers in solution state was evaluated. Based on shear stability analysis an inverse relationship was found between polymers’ shear stability and their molecular weights. Finally, the behavior of the commercial polymers was investigated for determination of optimum performance in engine cranking at-15°C. Based on the obtained data, the most suitable polymer was  identifed and  indicated as  the best  to fulfl oil  industry’s purposes. In this respect the high-tech polymers, based on their molecular structures demonstrate the best performance compared to other polymers. Paratone and Viscotech polymers have relatively shown similar behaviors, although based on viscometric behavior the Viscotech is better than Paratone, and based on shear behavior the order of preference is reversed. Keltan polymer has just a better thickening power while, it is not considered suitable for high quality engine oil at low temperatures of engine cranking for its weak shear performance and low effciency.