Evaluation of the fuel economy characteristics of engine oils requires a rigorous test design. Side-by-side test designs, though adequate for evaluating the control of deposits, wear, and viscosity, require too many cars for precise fuel-economy testing. Crossover, Latin-square, and other balanced-block designs cannot be used if the fuel economy oil shows “carryover” effects; that is, if the engine is conditioned by the fuel economy oil so that subsequent oils still show a residual fuel economy benefit.The most suitable test design is a sequential one. The reference oil is run until it demonstrates a constant level of fuel economy. The test oil is then run for enough miles so that its full effect has been reached. The comparison is made between the two stabilized levels. Examples are given for an SAE 10W-40, API SF oil, which showed a 4+ % fuel economy improvement in both fleet and EPA type chassis-dynamometer sequential-design tests.A “deconditioning oil” technology is currently being evaluated. The fuel economy of a vehicle is determined on a reference oil and a fuel economy oil in a sequential manner. The vehicle is then run for a short period on the deconditioning oil, which removes any friction-reducing films from the rubbing surfaces, and restores the engine to its original reference oil fuel economy level. If successful, this deconditioning technology will greatly reduce the time and cost of fuel economy testing.