There is a need for sensors that respond to chemical and physical properties of engine oil. In response to this need, an experimental design of an engine-mounted, capacitive sensor has been developed to monitor changes in the dielectric constant of the engine oil. The sensing element is a small, air-gap capacitor that is mounted in a spacer ring that fits between the oil filter and the engine block. Embedded in the same spacer ring is the associated circuitry.Experiments have been carried out with experimental capacitive oil sensors mounted on engines using a typical fully-formulated, factory-fill oil. The oil dielectric constant initially decreases and is less than the starting value while the anti-oxidant additives are active. After about 1600 km, the antioxidant additives become sufficiently depleted to allow oxidation products to accumulate and for the oil dielectric constant to increase at a steady rate of about 1% per 1000 km.