I wander, what would a oil quantity gauge change? Wouldn't more data only add to the confusion ? If the oil pressure drops, there is something wrong going on, and you should land ASAP, I don't think it really matter if the problem is leakage, faulty pump, or gauge, all of them pose a serious risk for the flight anyway.
There could be torque gauges added to the shafts to monitor friction between gears, and this could also show a lubrication system malfunction. But still... is there a need for deliberate complication of this system ?
On the other hand it actually amaze me, that in one hand we have a digital system that monitor everything, and in the other we have a strict algorithms to judge what to do in specific situations... and yet the pilot is the one who have to use them. Why can't the system, just tell what is going on ?
If the flight manual say: A:fail + B:ok + C:fail = land ASAP. Why can't the system do this on its own, and just put a sign on the MFD: "Oil system failure. Put me down NOW!" ?