Increasingly stringent fuel economy and emissions regulations around the World have forced the further optimization of nearly all vehicle systems. Many technologies exist for improvement in fuel economy; however, only a smaller sub-set are commercially feasible due to cost of implementation. One area that can provide a small but significant improvement in fuel economy is the lubrication system of an internal combustion engine. Benefits in fuel economy may be realized by the reduction of engine oil viscosity and the addition of friction modifying additives. In both cases, advanced engine oils allow for a reduction of engine friction. Generally speaking, the impact of chemical additives such as friction modifiers (FMs) is to reduce friction in tribocouples which experience metal-to-metal contact. These conditions commonly occur in valvetrain contacts and between the piston rings and cylinder bore at Top Dead Center (TDC). Lowered viscosity can reduce friction in journal bearings and the power-cylinder at mid-stroke. While these statements are largely correct, they are also highly dependent on specific engine architecture and operating conditions. For example, this work will show that there is a limit to friction reduction via engine oil viscosity reduction. Below a certain point, oil film thickness becomes insufficient to prevent metal to metal contact. In this case, friction will actually increase in certain tribocouples. Complicating matters further, different tribocouples may experience this increase in friction at different oil viscosities. To help understand how changes in engine oil viscosity and formulation impact friction in various tribocouples, a new test was developed. This test uses an electric motor to drive an un-fired test engine. Additionally, this test allows for the removal of hardware components allows for the isolation of various tribocouples for investigation with different oils. Using this data, together with data from previous chassis dynamometer vehicle testing and fired engine dynamometer fuel economy testing, an optimized engine oil is developed which provides the best balance of viscosity reduction and formulation to provide the highest fuel economy.