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The mechanical shear stability of polymer-thickened lubricating oils in relation to laboratory bench-scale techniques was investigated. In subsequent studies using standard laboratorytype paint blending-mixing devices, a kinetic dispersion mill (Model L) provided a mechanical shear environment demonstrating a directional trend of degradation for polymer thickeners. Further refinement of this technique led to the establishing of a correlation with single and multicylinder engine tests.Lubricating oils designed for use in automotive internal combustion engines can be classified into one of two types: conventional (a single grade) or polymer-thickened (a multigrade) oils. The conventional type crankcase oils are formulated using combinations of base stock blending fractions (light or medium neutrals, bright stocks, etc.) to fulfill the viscosity requirements specified for the individual SAE grade designations. However, multigrade engine oils are formulated using selected polymer thickeners in concentrations up to approximately 20% in a light base stock lubricating oil. The polymer concentrates (polymethacrylates, polyacrylates, polyisobutylenes, etc.) exert a thickening action which increases the viscosity by means of their solubility in the base lubricant (Lyman

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