This study focused on the effect of engine oils on regulated emissions, particulates and fuel economy. Three engine oils of the same SAE grade (synthetic oil with poly alpha olefins (PAOs), Group III base oil, and Group III genuine oil with additive package) were used in one gasoline and one diesel vehicle.A GDI (Gasoline Direct Injection) vehicle and a diesel vehicle without DPF (Diesel Particulate Filter) were selected because those vehicles obviously emit more particulates than port-injection gasoline vehicles and diesel vehicles with DPF. A combined mode consisting of the US EPA emission test cycles FTP-75 and HWFET was used for these tests. HORIBA and PIERBURG gas analyzers were used to measure regulated emissions and fuel economy, respectively. Unregulated emissions and particulates were analyzed by FTIR and PPM-S, respectively. Samples (300 ml) of test engine oil were taken periodically just after each test, and the colors of the sampled oil compared.The color of the engine oil samples became dark due to contamination. For the diesel vehicle, in particular, the sample color changed rapidly. The properties and components of engine oil comparing new and used one were almost same. Neither emissions nor fuel economy were seriously affected by engine-oil type in either type of vehicle. The number of particulates (PN) was similar with different engine oils, in both types of vehicle. The weight of the PAO particulate matter from the gasoline vehicle with synthetic oil was greater than that produced by other engine oils. The shapes of the particles from GDI and diesel vehicles were similar for all three types of engine oil, based on examination with a scanning electron microscope.