1. AbstractFuel economy is one of the most important performance features for modern engine oils. For some time now, fuel efficient engine oils (called Energy Conserving II or EC-II) have been available in the marketplace. However, the performance of EC-II oils is only 2.7% Equivalent Fuel Economy Improvement (EFEI) as measured by the ASTM Sequence VI Engine Test. To meet future industry needs, more fuel efficient engine oils are desirable.In order to achieve this, a study of highly fuel efficient engine oils was initiated. An initial target of 3.9% EFEI was selected and several candidate oils were evaluated, some of which exceeded this target. The oils were evaluated using a chassis dynamometer using the U. S. EPA mode. The test results may be summarized: 5W-30 Prototype Oil containing MoDTC showed between 1.6 and 2.6% better fuel economy than conventional 5W-30 and 10W-30 EC-II oils. There was an optimum viscosity for maximum fuel economy using the EPA testing mode. For MoDTC containing oils the optimum High Temperature High Shear (HTHS) viscosity was found to be 2.6mPa·s.In addition, a correlation was found between the Sequence VI and the chassis dynamometer test. However, the fuel economy improvement in the Sequence VI EFEI was about twice that of the chassis dynamometer. This difference is attributed to the higher oil temperature condition of the Sequence VI which results in more boundary lubrication. Friction coefficients of the 5W-30 Prototype Oil were measured with a Falex tester and the rubbing surfaces in the Falex test were analyzed using X-ray Photoelectron Spectroscopy (XPS). On the rubbing surfaces, MoS2 was formed resulting in a reduction of the friction coefficient.