Attending the oncoming licensing of API F4K oil category for Heavy duty Vehicles, which main objective is to embrace the opportunity to reduce fuel consumption and CO2 emissions lowering the High Temperature High Shear (HTHS) oil viscosity a fleet test was performed during 2016 in a fleet of urban buses in the city of Valencia. This paper describes the results of a comparison test where the effect of the use of API FA4 like engine oils are used to measure their fuel consumption benefits over the fleet. The aim was to verify and quantify the theoretical benefits in terms of fuel consumption in Heavy Duty applications. The study was performed using 48 urban buses which include diesel and CNG models of the city of Valencia. These buses were divided in two groups; a control and a test group, using regular oils and API FA4. The fuel consumption of buses was calculated with distance and refueling. Distance measurements were controlled by GPS and fuel consumption was measured by means of the buses refueling both of them in a daily basis.