One of my sisters recently bought a late-model used Saturn SUV recently.
I was surprised to hear her tell me that it has a sensor that operates between the oil pan and the oil filter.
It monitors the oil level and its “purity” - which would vary depending on amount use and type of use over time.
When the oil becomes lower than some level the sensor is looking for, or the purity is less than some level the sensor is looking for, a “change the oil” idiot light on the dash lights up, when the engine is running.
I’m not that fond of Saturn’s myself and I don’t know how good its oil sensor system is; I just didn’t expect it from them.
GM has put oil change lights in a lot of cars, starting, I believe, with mid 80s Cadillacs and perhaps Corvettes of the same era.
I chatted online with a GM engineer awhile back and he was pretty proud of the algorithms they developed. It’s not just a mileage counter. Apparently they take into account all sorts of inputs from the ECM to determine how often the oil needs changed based on driving style. Only thing they couldn’t monitor when I talked to him was the amount of environmental particulates, so he suggested those in extreme dusty environments might want to change more often. IIRC, the algorithms erred on the conservative side but they did help stretch the change intervals if your driving style warranted it.
It’s been a few years. It would be interesting if they had actually developed some sort of particulate detector since then.
The "oil condition" is determined by an algorithm running in the powertrain controller that considers a bunch of operating factors and calculates remaining oil life.