The 928 S4/GT/GTS takes about 7.5 liters of oil. Part of it resides in the engine, oil hoses, cooler, filter and other bits, but most of it rests in the oil pan. Below you see the oil pan. It contains the oil level sensor that reports a low oil level even prior to running the engine. It is not related to the oil pressure sensor. It uses a floater as you can see.
It is unknown at what level of oil the sensor triggers, but the difference on the oil dipstick between min and max level is about 1.5 liters of oil. I presume the sensor triggers at about min level.
Here's a section of the manual detailing the level sensor and how to test it:
Oil level sensor.pdf
Active Oil Level Sender (Switch)
Low oil level in the oil pan (before starting the engine) is indicated by the “Check oil level” warning on the display. In addition, the new warning system displays a “Check oil level” warning in each of the following cases:
0- interrupted power supply to the oil level sender
1- faulty oil level sender (switch)
Oil Level Monitor
Up to the present, the oil level switch used for monitoring of the oil level was only checked with the engine stationary.
With the engine running, the engine oil is circulated throughout the entire lubrication circuit so that precise measurements of the oil level are not possible in this mode.
These characteristics are now used to check the oil level monitor:
The switching state of the switch is now also inquired with the engine running, depending both on speed and time.
If no change of the switching state occurs within a predetermined span of time, the warning “Check oil level” is displayed.
If it is found that the oil level is o.k., the operation of the oil level switch must be checked. Two faults may cause the “Check oil level” warning to be displayed:
- oil level too low, or
- oil level switch or signal path faulty
The mileage covered is stored in the instrument cluster for both causes.
To check the oil level switch, use the “On-Board Diagnostics” and the “Check switch” feature.