A method for assessing the deterioration of motor oil is provided, in which the oil viscosity of the motor oil is measured by a sensor, and the length of an oil-change interval is determined in an evaluating electronics having a display device. According to the present invention, it is provided that a temperature sensor for the simultaneous determination of the oil temperature is assigned to the sensor for measuring the oil viscosity, and the oil viscosity and oil temperature are measured in the cooling phase of the engine after it has been switched off.