Government regulations and market demands continue to emphasize conservation of fossil fuels in the transportation industry. As a consequence, any incremental improvement in fuel economy (FE) is of great importance in the automotive sector. For instance, lower viscosity lubricants have been shown to improve FE but the longevity of such improvement is compromised by viscosity increases often observed as a lubricant ages during an oil drain interval (ODI). To address this issue, an option to manage lubricant viscometrics via fuel is proposed. In order to investigate such mitigation of viscosity increase during an ODI, and potentially the delivery of an ODI-averaged FE benefit, a fleet test was conducted with a fuel-borne additive intended to control increases in lubricant viscosity.The fleet test compared a market-representative reference fuel to a fuel containing a viscosity control additive (VCA). Five different European vehicle models were tested over a 15,000 mile ODI using a “quad” fleet testing protocol previously described. The FE evolution of each vehicle was determined by the NEDC (New European Drive Cycle) procedure at intervals throughout the fleet test and supported by a range of snapshot oil analyses.The fleet test showed the VCA fuel was able to mitigate lubricant viscosity increase and deliver an increasing fleet-average fuel economy improvement with respect to the reference fuel throughout the trial. Consequently, the benefits of both lubricant viscosity control and associated FE were greatest at the end of the trial where the reference fuel vehicles showed the greatest lubricant viscosity increase. These observations were supported by increasing concentration of VCA within the lubricant of the VCA-fuelled vehicles and by modeling of vehicle FE in conjunction with lubricant-VCA blending considerations. Subsequent lubricant and engine analyses showed no evidence of undesired effects from the VCA (e.g. enhanced wear rates).It is concluded that the fleet test demonstrated that VCA fuel additive package can limit the viscosity-increase tendency of engine oil during its lifetime and so enhance longevity of fuel economy performance.