Due to recent legislation on CO2 emissions, Heavy Duty engine and vehicle manufacturers and their suppliers have had an increased interest in improving vehicle fuel economy. Many aspects are being investigated including vehicle aerodynamics, tire rolling resistance, waste heat recovery, engine fuel efficiency, and others. Crankcase oils offer a cost-effective mechanism to reduce engine friction and increase engine fuel efficiency. The potential gains realized by optimized fuel-efficient oils are relatively small, usually less than 3%. Therefore, in order to develop these oils, formulators must have a robust, repeatable, and realistic test method for differentiation. To serve Light Duty (LD) engines, this need has been partially satisfied by the development of what became the Sequence VI engine test for gasoline passenger car oils in the early 1990’s. More recently, several engine based FE programs have been developed to support the Heavy Duty (HD) and Medium Duty (MD) diesel industry. Results and observations from a selection of these programs are presented below that include work on the following engine platforms: Cummins 5.9L and 15L, CAT C13, and Volvo MP8. Realizing and quantifying small improvements in fuel consumption requires attention to detail in the set-up and execution of the test procedure in order to report results with confidence.This paper discusses some of the key elements to a repeatable test procedure, including instrumentation, control, cycle design, and specific engine effects. Often times, the most challenging variable is the engine itself.