Method and system for reducing fuel consumption in a diesel engine

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.