The physical characteristics of lubricating oils can affect the mechanical efficiency of internal combustion engines and as a consequence the quantity of fuel consumed at constant power output; moreover they can influence the quantity of lubricating oil drawn through pistons and valves towards the combustion chamber.Exhaust emissions can be influenced both by fuel and oil consumption, so the physical characteristics of the oil can indirectly affect them.The purpose of this experimental work is to study the effect of the physical characteristics of lubricating oils on emissions, fuel economy and oil consumption in a light duty diesel engine.The study was carried out on a VM Turbotronic diesel engine characterized by low exhaust emission levels (1996 European limits for passenger car achieved without catalyst) and high specific power (37 kW/litre).Tests were carried out both on engine dynamometer, according to non-standard cycles, and chassis dynamometer, according to European standard cycles ECE15 and EUDC, using different lubricating oils. The results showed the important role of oil viscosity in limiting fuel consumption: a significant decrease was noticed in particular in urban running condition using low viscosity oils; this reduction gave a correspondent decrease in carbon dioxide emissions. The tests on an engine kept in the standard configuration of its electronic control unit pointed out an ambiguous effect of rheology on exhaust emissions: low viscosity oils gave higher HC and CO emissions but guaranteed lower NOx emissions. Total particulate seems not to be affected by oil rheology, but the contribution of lube and fuel to the Soluble Organic Fraction (SOF) of particulate could depend on rheology: low viscosity oils seem to decrease the lube contribution to SOF but they seem to increase the fuel contribution.Tests carried out in the same running conditions but optimizing the engine for each tested lubricant by the modification of injection timing and EGR rate, showed the possibility for a fluid oil to produce the same polluting emission of a viscous one by means of a slight increase in EGR rate and injection advance, thus conserving the advantages of fluid oils in terms of fuel economy and carbon dioxide emission without detriments. Moreover, tests at high power output showed the possibility of limiting oil consumption as well as sump oil temperature by using low viscosity oils.