Given the influence that oil viscosity has on the engine, a technique was developed to calculate the effects of engine oils on fuel efficiency. To be meaningful, the viscosity values had to be obtained at the high shear rates associated with operation in specific sections of the engine. 


Earlier dynamometer work had identified the percentage of friction and operating temperature of the five main lubricating sites in a reciprocating gas-fueled engine responsible for nearly all efficiency loss. This information was used to develop the viscous fuel efficiency index (V-FEI) parameter. With this value, which ranges from 0 to 100, the higher the V-FEI of a given engine oil, the less energy is lost to viscosity, and consequently, the more fuel efficient the engine is. Although different engine designs may have different levels of friction in the essential lubricating areas, use of this friction data provides a comparative value for engine oils.


Figure 3 shows the average value of SAE 0W-20 and 5W-30 engine oils from the North and South American markets from 2008 to 2014. For comparison, the average V-FEI for SAE 0W-20 and 5W-30 in an earlier study was 46 and 47 respectively.


As expected, it was determined that the yearly averaged multigrade SAE 0W-20 oils contributed more fuel efficiency to the engine than did the averaged multigrade SAE 5W-30 oils because of the viscosity differences shown in Figure 2. With the exception of 2012, the increase in V-FEI is equivalent to nearly 7 to 8 percent in viscosity-dependent fuel efficiency. The decrease shown in the average fuel efficiency of SAE 0W-20 engine oils collected in 2012 may indicate the development of formulations meeting automakers’ concerns that the benefits of hydrodynamic lubrication will not be lost in efforts to improve fuel efficiency.