A comparison is made of fuel and engine oil consumption of diesel highway maintenance trucks with 27,000 lb gross vehicle weight using standard 15W-40 engine oil and synthetic engine oil. A control group and a test group, of eight trucks each, madeup the test fleet, which was monitored for a period of 18 months under normal operating conditions. The trucks were geographically distributed throughout the state of Maine, so that one truck from the control group and one from the test group were located in each of eight geographic regions. For the entire 18-month test period, the data indicated that the trucks using synthetic oil had a 5 percent overall improvement in fuel consumption rate (fuel used in gallons per hour of operation) over the trucks with 15W-40 oil. Fuel consumption data were also compiled separately for the summer and the winter. The fuel consumption in gallons per hour for the test units during the summer was approximately 4 percent less than that for the control units. For the winter period the test units exhibited a fuel consumption advantage of approximately 7 percent over the control units. The wear analysis of the engine oil indicated there may be more wear when the synthetic oil is used.