The quantitative determination of aging effects on motor oil is of great interest. According to a method, two variables of the motor oil are detected, one of the two variables being the viscosity and the other variable being the acid content of the motor oil. A first deviation of the detected first variable from a default value is then determined, and the second variable is estimated based on the deviation. By determining a second deviation of the estimated second variable from the detected second variable, the second deviation is determined as a measure of the aging effect on the motor oil. Since various aging effects influence the two variables in different manners, individual aging effects may be quantitatively determined in a targeted manner.