The rheological behavior of dilute and semi-dilute solutions of stearyl methacrylate-co-styrene-co-vinyl pyrrolidinone copolymers (SNIA-S-VP) was investigated over a wide range of temperatures (20-140degreesC) under steady shear (0.1-5000 s(-1)) and dynamic conditions (0.01-500 rad/s). The solvent used was polyalpha olefin, PAO4 (C20-C40 fraction), a fully synthetic paraffinic oil normally used as base oil for lubricant oil formulations. The investigation focuses on the effect of the polar comonomer segment content, of the copolymer, on the viscosity-temperature relationship of the solution. This information provides insight into the effect of polymer n-Acrostructure on the viscosity index (VI) behavior of. the lubricant. Copolymers with different ratios of stearyl methacrylate, styrene and vinyl pyrrolidinone were synthesized by solution radical polymerization. The microstructure and molecular weight of the copolymers were evaluated by nuclear magnetic resonance spectroscopy (NMR). The molecular weight and polydispersity of the polymers were determined using GPC. The polymer solutions exhibit non-Newtonian behavior, which is more evident at lower temperatures and higher concentrations. The temperature-concentration superposition principle developed by Ferry (1) was applied to steady shear data and the resulting master curve was fitted to the Carreau model. A modified Fedors equation for intrinsic viscosity calculation, at low and moderate polymer solution concentrations, Was used. The results confirm that the viscosity index improver properties (VII) of the copolymer are superior when the VP content is low. The polymer-solvent interactions and solvency power of PAO4 were analyzed using an Arrherrius expression. The results show that at low VP content, PAO4 is a better solvent for the copolymer. (C) 2004 Society of Plastics Engineers.