The quality and/or aging of an oil is monitored, notably in-situ, using a resonator sensor, for example a quartz crystal microbalance sensor (QCM). Changes of the frequency response of the sensor may be used to detect or monitor the presence of one or a combination of water, fuel, metallic debris, plastic debris and reaction products from degradation of oil components and/or oil additives (including oxidation products and acids). At least one of the major surfaces of the resonator sensor may include a component capturing surface layer provided with impression sites adapted to retain components of the oil indicative of the oil's quality, aging or contamination and both major surfaces of the sensor are preferably immersed in the oil to be monitored.