[J.C.]

Quickly skimmed over the thread.
I'm with Cam and Mik (and whoever else suggested similiar). Purchase a gauge, tap it into the oil gallery and see what is truely happening. Grab a sensor as well and replace it if you end up finding it reading incorrectly........or just throw one at it anyway as they do fail. They are cheap enough.
Also wish to back up the suggestion of low system voltage at idle. Do not disregrade this. It is the cause of many a non existence faults and errors within electrical circuits. Leave a car or machine idling long enough and the current and voltage draw is higher then the alternator output at low RPM.
Idle speed? Is it to low due to parasistic loads such as worn and dragging pulley bearings for example, blah, blah, blah.
Heat soak plays havic with cars and stationary machinery where it's principle design was to have movement to remove heat and not souly rely on water jacket water or some ****y thermo fans.

If you can't find anything simple out of the suggestions in this thread, purchase yourself a new filter and filter cutter. Inspect engine oil filter ............with fingers crossed. Remember, inspect the outside of the filter. Not the inner area. Filters outside in, through the filter. Cut a section of the filter out once you have the element apart and squeeze excess oil out of it (use a vice if you have one available). Open it up and read it like a book. It tells a story.
My experience with AU 4L's is they are generally pretty bullet proof and screwed together well enough for a mass produced factory engine. But, like everything, they are only man made and components, sensors etc do fail.
Let us know how you go. I'm keen to hear the outcome. I sincerely hope it starts and stops with a simple engine oil pressure sensor.
Cheers.
JC