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A Gel Permeation Chromatography-Fourier Transform Infrared (GPC-FTIR) technique is employed to monitor changes in the molecular weight distributions of polymeric oil additives caused by oil aging in vehicle and engine fuel economy tests. Before and after oil aging, the predominant high molecular weight polymers in the oil are the dispersant and viscosity index improver. That is, very few low molecular weight species are oxidized and subsequently polymerized during the fuel economy tests. Molecular changes in the dispersant and viscosity index improver are related to changes in an oil's high temperature high shear viscosity in order to determine their effect on an oil's ability to control fuel economy.