Oil level is tricky to measure in a car on the move as the oil sloshes around a lot, so the system has to take an average and balance between averaging over a long period, i.e. reacting slowly, which could mean that a warning comes too late, and reacting quickly, which will mean more false danger signals, e.g. going up a long hill. A reliable oil level warning has the advantage that it will warn you before damage has occurred.
Oil pressure is relatively easy to measure, and should be stable between oil pump and bearings. However, low oil pressure warnings are usually a sign that some damage has already occurred.
Almost all cars are fitted with an oil pressure sensor, usually wired to a dashboard annunciator light. Very few cars have an oil level sensor, which is always in addition to the pressure sensor.
My old Golf GTI had two oil pressure switches, the second being a low pressure switch on the head, detecting pressure to the camshaft. This one would always start to beep on fast roundabouts when the oil was getting low, indicating that oil was sloshing away from the pickup pipe. The high pressure switch never came on, though (they had different beeps and indicator light patterns). It was a good system, but probably considered too confusing for the average driver. Sometimes I think I don't want to meet the average driver - other than avoiding him on the roads every day, that is.
I think all Leons have the oil pressure sensor attached to the oil filter bracket, which is attached to the engine block and not part of it. So there is still hope if you knacker the threads on the filter bracket. Of course, you then have to get the filter-bracket-to-block bolts undone . . . . .
