A system includes a resonant sensor in contact with oil within a gearbox of a rotor system, such as a wind turbine, and one or more processors. The sensor includes electrodes and a sensing circuit that generates electrical stimuli having frequencies applied to the oil at different times during a life of the gearbox. The processors receive electrical signals from the resonant sensor representative of impedance responses of the oil to the electrical stimuli. The processors analyze the impedance responses and determine a concentration of a polar analyte in the oil at different times. The processors calculate a degradation value for the gearbox based on the concentration of the polar analyte. Responsive to the degradation value exceeding a designated threshold, the processors at least one of schedule maintenance for the rotor system, provide an alert to schedule maintenance, or prohibit operation of the rotor system until maintenance is performed.