A diesel cycle internal combustion engine can be operated with a variety of fuels having different boiling points and different viscosity-temperature characteristic curves, with a fuel circuit that includes a fuel tank, a fuel pump, a fuel line, at least one fuel filter, a high-pressure pump, an injection system device and a return line to return fuel to the fuel tank, where a control valve is provided in the return line for dividing or diverting the fuel flow to the fuel tank and/or to the fuel filter, and where the fuel division or diversion is undertaken as a function of the viscosity of the fuel in use such that the fuel filter will thence be heated to a temperature based on heat transfer from the fuel in use, and further where a pressure relief valve is provided between the fuel filter and the high-pressure pump.