Volumetric properties and viscosity of mixtures of a synthetic poly(alpha-olefin) base oil with polymethacrylate-based polymeric additives have been studied. Two different polymeric additives were evaluated at addition levels up to 7 wt %. Density data were collected at 298, 323, 348, 373, and 398 K from 10 to 40 MPa in a variable-volume view cell. Viscosity data were collected at 298, 323, 348, and 373 K from 10 to 40 MPa in a high-pressure rotational viscometer. The density and viscosity data were then modeled simultaneously by coupling the Sanchez-Lacombe equation state with the free volume and the density scaling theories of viscosity. Density data were also used to evaluate the derived thermodynamic properties such as isothermal compressibility, isobaric thermal expansion coefficient, and internal pressure. The variations in the volumetric properties and viscosity are discussed in terms of the nature of the polymeric additive and polymer concentration in the mixture. It is shown that for each oil, density scaling reduces the viscosity values generated at different temperatures and pressures into a master curve. Density scaling parameters provide new insights into the effectiveness of the polymeric additives as viscosity modifiers in terms of addition levels and the temperature sensitivity of viscosity.