One other note...An engine monitor, like the EDM830 we have fitted is also very valuable. If the Oil P went zero yet the EDM showed normal, then I'd consider it an indication problem.
Likewise if (for example) one cylinder starts to get hot or oil temp exceeds the pre-defined red-line limits we entered (or any other paramater being monitored exceeds a max or min limit) then we have a big red light which starts flashing on the panel. We can then cross check the EDM with primary instruments and if both agree then it is very quick to realise that there is an actual problem rather than a gauge problem. I highly recommend a decent engine monitor.
(plus you can download the data after a flight and keep a historical trend of each parameter. Useful for examining you engine's health over time).