This paper describes the performance of a synthetic diesel engine oil formulated to satisfy the most demanding lubrication requirements of modern heavy-duty diesel engines designed to meet North American and European emission regulations. The combination of an advanced fully synthetic base stock system and a customized additive system has resulted in an SAE 5W-40 oil with unique performance characteristics which include exceptional low and high temperature properties, excellent engine performance in laboratory and field tests, and an independently-documented, measurable fuel economy benefit relative to conventional mineral-based multigrade diesel engine oils. In addition to the cold starting and low volatility benefits derived from the synthetic base stocks, this technology has demonstrated outstanding engine performance in the areas of soot dispersancy, wear protection, engine cleanliness, and oil consumption control. This synthetic diesel engine oil exceeds the requirements of the API CH-4 and ACEA E4/E5 categories, as well as a number of more demanding North American and European engine builder specifications. Engine performance in the laboratory tests required to satisfy these specifications is presented, as well as extended oil drain interval field trial experience. Additionally, controlled commercial vehicle fuel economy test results documenting the benefits of this synthetic diesel engine oil are summarized.