The chemical and/or physico-chemical determination of the ageing of a motor oil is very expensive and requires measuring methods which cannot be carried out on board a motor vehicle. The determination of only one oil condition parameter, for example the viscosity, only enables conditional statements to be made with respect to oil quality since conflicting effects may exist to varying degrees in this case. According to the invention, oil quality can be determined by a quartz base coated with a sensitive layer. The sensitive layer has a surface or volume which is adapted to an oil component and is suitable for the repeated incorporation and release of the oil component according to the concentration thereof. When the oil component is present, it is incorporated in the sensitive layer causing the resonant frequency of the layer to decrease via a mass effect or causing an effective increase of the component thickness or mass. As the oil ages, the proportion of the component incorporated in the sensitive layer decreases, so that the resonant frequency increases. A non-sensitive layer is used as a reference by which the viscosity effect of the oil on the oscillation of the quartz and the variation in viscosity of the motor oil are determined as a second important oil quality parameter.