This paper shows the results of a fuel consumption in-use comparison test where the effect of Low Viscosity Oils (LVO) was evaluated over a sample of 39 urban buses powered by Diesel and CNG engines. The aim of the test was to verify the fuel consumption benefits of LVO in Heavy Duty Vehicles (HDV) found in previous works, which were obtained mainly in engine test bench, when engines are working on “on-Road” conditions. In order to achieve this goal, a sample of 39 urban buses was studied over an Oil Drain Interval or 30.000 km (approximately an 11 month period), measuring daily mileage and fuel consumed to calculate each bus fuel consumption. Mileage was measured by GPS and fuel consumed was measured from refueling system.The sample was divided into two groups; a control group of buses using reference oils (SAE grade viscosities of 15W-40 and 10W-40) and a candidate group using LVO oils (SAE grade viscosities 5W-30). As kinematic viscosity at 100°C and High Temperature High Shear (HTHS) viscosity at 150°C have shown a good correlation with engine fuel consumption, an oil sampling program was implemented to study the possible variation of these oil parameters and its effect on the fleet fuel consumption during the test.Results indicate a positive correlation between the use of LVO and fuel consumption reduction in HDV, both for Diesel and CNG engines.