Advancement in Heavy Duty Diesel Engine Oils has, for approximately two decades, been driven by the ever more stringent emission legislation for NOx and Particulates. Over the last few years, the focus has shifted to reducing CO2 emissions and reducing operating cost by improving the engine's fuel economy.With fuel economy as an important new technology driver, the industry is exploring and introducing diesel engine oils of viscosity grades that used to be applied solely in passenger car engines, such as SAE 10W-30 and even SAE 5W-30. To avoid misapplication, API has decided that heavy duty diesel engine oils, most of which are formulated close to the maximum 0.12% phosphorus limit in the API C specification, can no longer add the API S gasoline engine claim. The only way to create a lubricant that carries both an API C and S claim for mixed fleet or municipality application, is to formulate at less than 0.08% phosphorus, a limit that was adopted in API S specifications because there are indications that phosphorus may foul three-way catalysts used with gasoline engines to control tailpipe emissions.And there lies the dilemma. The market wants to move to lower viscosity grades and maintain all the capabilities that current diesel engine oils exhibit, including universal applicability in both diesel and gasoline engines, and the robustness that is required for extended drain capability and engine durability. The conclusion seems obvious: A real performance upgrade to maintain engine durability despite lower viscosity and a lower phosphorus limit, is necessary. This publication describes the steps that were taken to successfully meet the seemingly contradicting demands of tomorrow's diesel engine oils, and thereby takes the performance of a new generation of diesel engine oils to the next level.