Engine Oil Temperature Sensor
The engine oil temperature sensor is located in the oil filter housing on MPi engines and in the Hydraulic Control Unit (HCU) on VVC engines. The sensor provides a signal which allows the ECM to adjust fuelling values according to engine oil temperature, to produce optimum engine performance and minimum emissions during the engine warm up phase. On VVC engines, the ECM also uses the oil temperature to derive the viscosity of the oil passing through the HCU, which indicates how quickly the VVC mechanism will respond. The engine oil temperature sensor consists of an encapsulated Negative Temperature Coefficient (NTC) thermistor which is in contact with the engine oil. The engine oil temperature sensor operates in a similar manner to the ECT sensor. If the engine oil temperature sensor fails, the ECM will substitute a default value which is ramped up 80°C (176°F). This condition will not be apparent to the driver, with the exception of the temperature gauge which will display incorrect readings depending on the sensor failure.
The vehicle will run but may suffer from reduced engine performance and increased emissions as adaptive fuelling is disabled. The ECM will store fault codes which can be retrieved using TestBook.

... but now I've checked I see that the post '99 uses one dual element sensor to feed both the Engine ECU and the gauge.