Viscosities were measured on 1% solutions of olefin copolymer (OCP) VI improvers in oils using a variety of techniques to obtain comparisons under different shear rates. OCPs with 60-70 mole % ethylene were amorphous; those with 80 mole % contained small amounts of crystallinity.Kinematic viscosities at 40 and 100 ° C increased with molecular weights, regardless of OCP composition. Haake Viscometer data showed that at room temperature and low shear rates solutions of all copolymers were Newtonian. When the temperature was lowered, amorphous copolymers remained Newtonian while partially crystalline OCPs became non-Newtonian and gave viscosities that were much lower than expected from molecular weight. At -20 °C, high shear rates in the CCS reduced the dependence of viscosity on OCP molecular weight and viscosity changed very little with increase in Mw from 106,000 to 336,000; at low shear, however, in both the MRV and Haake, viscosities increased appreciably as Mw rose. Under all shear rates investigated by us, low temperature viscosities of the partially crystalline OCPs were below those of amorphous OCPs with similar molecular weights. These results support our suggestion that in oil solutions at low temperatures the structure of partially crystalline OCPs facilitates the growth of ordered polymer domains which lower viscosities.