As a result of coming government regulations, the sulfur content of diesel fuel for on-highway use in the United States will be reduced from the current typical of 0.25 mass % to 0.05 mass % maximum. This paper describes a field test using this lower sulfur fuel in commercial heavy-duty engines to determine any potential benefits or harm to engine performance. Further, a lubricating oil with 0.5 mass % sulfated ash designed, to perform with the new low sulfur fuel, was evaluated in this field test and compared with a current premium commercial API CF-4/SG oil with 1.0 mass % sulfated ash. The field test also compared these oils using current regular diesel (0.25 mass % sulfur) as control.This paper presents the interim results of this field test at 480000 km (300000 MI.). No performance debits were seen in the test engines as a result of using low sulfur fuel. When using the premium conventional API CF-4/SG oil, there was little difference in engine cleanliness when using low sulfur fuel compared to normal diesel fuel. Engines using low sulfated ash heavy duty lubricating oil and low sulfur fuel showed a significant decrease (benefit) in piston crownland carbon formation and overall cleaner piston ring belt zones. Further, the low sulfated ash oil was fully equivalent to the premium conventional 1% sulfated ash oil in engines using normal (0.25% sulfur) diesel fuel.Overall, the interim results from this field test show that no harm is seen in heavy-duty diesel engine operation in going to appropriately formulated fuels with significantly lower sulfur content. Furthermore, current lubricant additive technology can make a lower sulfated ash engine oil to take advantage of the reduced fuel sulfur content.