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This paper illustrates how experimental design techniques can be employed to obtain an accurate assessment of multigrade Heavy Duty (HD) performance in the field. Compared to monograde (SAE 40) oil, the use of SAE 15W/40 oil was found to improve vehicle fuel mileage (MPG) as well as oil economy (MPQ). The demonstration, conducted in two groups of 10 city buses each in regular service, was structured to dampen out the effect of numerous incidental variables. Statistical data analysis indicate a “most probable” improvement in fuel mileage of 2.7% as well as a 47% increase on multigraded oil economy.