Originally Posted by JCL

Yes, if the OBD did not function you could get a catastrophic failure. It is likely designed to be failsafe, which means that if the OBD fails the engine stops. However, even a failure of the dashboard display light could lead to you driving it without realizing that the warning was there.

My comment on PDC was that driving it until the light comes on is similar to those people who park by sound. I am not referring here to the ones who have PDC, with an audible distance warning, but rather those who back up until they hear contact with a car behind them. The point is that it is usually better to stop just before that. With oil level, that just means checking it regularly when you fill up. Once you know how much oil it consumes, you will know how often to check it, ie the consumption is likely to be somewhat linear.

Even if you don't run it right down to the warning light, running a few litres low isn't as good for the engine as keeping it topped up.

I believe the three levels of warning are amber for low oil (50 km to get some oil in it), red for low oil (get some oil in it right now) and red for oil pressure (the oil pump just drew air, and it is time to listen for those expensive noises that indicate bearing failure). The first light isn't the end of the world, but OTOH there is a reason the vehicle has a dipstick.

And don't beat yourself up too much, because there are many people who never check the oil. That is what led to the inclusion of the additional lights for oil level, before the traditional oil pressure warning activates. It has gone even further with the new models (E90, etc) that don't have a dipstick and rely totally on warning lights, albeit slightly more sophisticated ones with a level readout on dash.