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This paper investigates the phenomena of gel formation and poor filtration performance of engine oils containing certain olefin copolymer (OCP) viscosity modifiers (VMs). Low temperature pilot plant and bench scale filtration tests have been used to study the filtration behaviors of a wide range of low-ethylene and high-ethylene OCP VMs at various temperatures but with particular focus on the performance at 0°C. Further, rheological studies have been carried out on fully formulated oils as well as on PAO solutions of these polymers to understand the effect of thermal history on the polymer solution behavior over a broad range of temperatures. These studies suggest that many high-ethylene OCPs are prone to the formation of polymer aggregates in oil solutions. Long crystalline polyethylene segments present in high-ethylene OCP molecules are believed to interact with similar segments in neighboring molecules, leading to the formation of aggregates.