The viscosity behavior of diluted, semidiluted and concentrated solutions of maleic anhydride (MA), dodecyl methacrylate (DDMA), and octadecyl methacrylate (ODMA) terpolymers in mineral oil was investigated. Terpolymers with MA molar ratio of 5 and 10% were synthesized by free radical polymerization in mineral base oil. All the terpolymer solutions in base oil exhibited high kinematic viscosity and values of viscosity index (> 140), proving very high thickening efficiency. Increasing the MA content in terpolymer decreases solution viscosity. Viscosity-concentration dependences of dilute solutions at 40 and 70A degrees C were described by Huggins' equation, and intermolecular interaction were discussed. In comparison with conventional polymeric viscosity index improvers based on the pure alkyl methacrylates, copolymers of long-chain alkyl methacrylates with small amounts of MA show improved shear and thermal stabilities.