More stringent emissions regulations, fuel economy standards, and regulations are currently being discussed to help reduce both CO2 and exhaust emissions. Vehicle manufacturers have been developing new engine technologies, such as downsizing and down-speeding with reduced friction loss, improved engine combustion and efficiency, heat loss recycling, power-train friction loss recycling, and reduced power-train friction loss.The use of more efficient fuel economy 5W-30 engine oils for heavy duty commercial vehicles has started to expand since 2009 in Japan as one technological solution to help reduce CO2 emissions. However, fuel economy 5W-30 oils for use in heavy duty vehicles in Europe are mainly based on synthetic oils, which are much expensive than the mineral oils that are predominantly used in Japan.The main key technologies for mineral oil-based 5W-30 engine oils are the additive technologies used to determine the optimum conditions for the friction modifier, viscosity modifier, and HTHS viscosity in accordance with the lubrication conditions of the bearings, pistons, cylinders, and valve train on the Stribeck curve. A mineral oil-based 5W-30 engine oil is also required to have excellent detergency and anti-wear performance.This paper describes the evaluation of fresh engine oils using engine tests, with a particular focus on the viscosity index and HTHS viscosity to help determine the effect of these oils on improving engine fuel economy. The HTHS viscosity was evaluated at 150°C/107s−1 instead of the typical 150°C/106s−1.This paper also focuses on the boundary lubrication conditions and ways to improve fuel economy using the friction modifier. The fuel economy improvement effect was evaluated through engine tests after first confirming the friction reduction effect of the friction modifier through rig tests. Furthermore, an additional evaluation was carried out using carbon black to evaluate the effect of used oil under actual usage conditions.This research is expected to help contribute to the further expansion of the use of mineral oil-based 5W-30 engine oil in heavy duty diesel engines.