The present invention relates to a method (and corresponding system) of reducing fuel consumption in a diesel engine (300) when operating at less than full power, the method comprising the step of: adjusting (101) a viscosity of a system oil (301) from the engine (300) resulting in a modified system oil (301′), where the viscosity is adjusted so that the mechanical efficiency of the engine (300) is increased when the modified system oil (301′) is provided to the engine (300) and wherein the step of adjusting is done on the basis at least one parameter representing an actual load of the engine (300) and a viscosity parameter of the system oil (301). In this way, the mechanical efficiency of the engine when operating at less than full speed/at partial speed is increased. This reduces the fuel consumption of a diesel engine and reduces the emissions of a diesel engine.