Depending on the vehicle manufacturer and the specific equipment used, oil indicators come in two basic varieties: algorithm-based and direct measurement.
Algorithm-based oil indicators measure lots of factors and then plug the resulting numbers into a formula. Based on the answer to this complex, ongoing math problem, the indicator display will tell you whether the oil is OK, is close to requiring replacement or needs replacing immediately.
Interestingly, with these types of indicators, there are no sensors to detect the quality of the oil itself. Instead they combine data on how many miles you've driven, the temperature variations during that time and data about how much work the engine has performed. Typically, the indicator (monitoring system) will receive such data from the
powertrain control module, or
PCM, which is the main on-board computer. Engineers have figured out a fairly accurate and reliable way to calculate the remaining oil life this way, without having to actually sample the oil.
Direct measurement oil life indicators measure the condition of the oil -- the opposite approach to the system described above. This method uses sensors to sample the oil and determine its remaining life based on any of the following:
- Conductivity -- how easily electric current passes through the oil (typically, the lower the electrical resistance, the more contaminants are in the oil)
- Mechanical properties -- piezoelectric sensors can tell how thick the oil is by the force feedback it gives when sloshing around
- Soot concentration -- dirty oil's days are definitely numbered
- Presence of water -- water is an impurity in oil, since it hampers the oil's effectiveness and can corrode metal surfaces