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Miscibility and rheological properties like kinematic viscosity at 40 °C and 100 °C, viscosity index, shear stability and pour point for solutions of polyolefin copolymer (OCP) and terpolymer poly(styrene- co -dodecyl methacrylate- co -octadecyl methacrylate) (PSAMA) in lubricating mineral oil are examined. Additives show complementary effect in which OCP increases solution viscosity while PSAMA increases viscosity index and lowers pour point. Repulsive interactions of OCP/PSAMA in oil solutions were determined by Krigbaum–Wall and Catsiff–Hewett models. Calculations employing the group contribution approach of Coleman, Graf and Painter confirmed immiscibility (Λ 12 > 0, Δ G M > 0) of OCP/PSAMA blends. The obtained results give the prospects for lubricant formulations with mixed polymeric additives as well as for lubricant handling in order to avoid separation of copolymers in solution. [ABSTRACT FROM AUTHOR]