A method for determining a fuel fraction in the oil of an oil circuit (6), in particular in oil of an oil circuit (6) of an internal combustion engine (7), wherein a variably adjustable oil delivery pump (1) is provided in the oil circuit, which oil delivery pump can be actuated by means of a control unit (4), wherein the control unit performs an adjustment of parameters and/or adaptation values of the oil delivery pump as a function of the viscosity of the oil, wherein a determination of the fuel fraction in the oil is performed on the basis of the parameters and/or adaptation values.