Viscosity modifiers (VM), also often called as viscosity index improvers (VII), are polymers used as additives in lubricant compositions for minimizing the variations in viscosity of the lubricant as a function of temperature. The degradation stability towards mechanical shearing (shear stability) of three types of polymers, namely polymethacrylate (PMA), olefin copolymer (OCP) and hydrogenated styrene-isoprene copolymer (SIP) which are generally used as VMs in lubricants, has been investigated at varied polymer concentrations (0.5 to 4.0% wt/wt) in five high viscosity index (HVI) and one medium viscosity index (MVI) category of mineral oils using a Bosch injector shear rig and ASTM D-3945 method. The effects of mineral oil (base stock) viscosity and the composition of the base stock as well as that of polymer concentration on shear stability Index (SSI) were primarily studied. The effect of polymer concentration on SSI was distinctly observed and its magnitude was dependent on polymer chemistry and the nature of base oil. The studies were extended to determine shear stability of these polymers in terms of high temperature, high-shear (HTHS) viscosities and it was,found that there is no correlation between SSI values based on kinematic viscosities and those based on HTHS viscosities. Studies were also briefly extended towards an understanding of the relationship between thickening ability of the polymers and their shear stability.