The properties of styrene/dodecyl methacrylate/octadecyl methacrylate terpolymers as viscosity index improvers of lubricating mineral oils were investigated. Terpolymers of different composition and molar mass were prepared by peroxide-initiated polymerization of the monomer mixture in a mineral base oil solution. All the terpolymer solutions in base oil exhibited high kinematic viscosity and viscosity index values (> 130), proving their high thickening efficiency. The ratio of viscosity values at 100 C and 40 C of styrene/alkyl methacrylate additive solutions was between 0.95 and 1.0, showing the coherent thickening effect in a wide temperature range, comparable with pure alkyl methacrylate additives. Increasing content of styrene in the additive and decreasing molar mass of the terpolymer resulted in a decrease in viscosity and viscosity index while the shear stability increased. The obtained results give guidelines for optimization of the terpolymers composition and structure in order to assure the best lubricant application properties.