Thanks for the heads up. I have read of the oil pump priming issue on +8`s after an oil change, thus that is why I asked... Best avoided by filling up the oil filter with engine oil prior to fitting, all a bit messy but has always worked on my mid 80`s +8.
I suspect you can discount the oil pressure relief valve sticking opened, unless when you last fired it up from cold, you reved the engine high enough fort the oil pressure to rise enough to lift the relief valve off it`s seating and it stuck opened.... which I suspect to be very unlikely..?

The screen in the pickup would only likely to be restricted if there was a high degree of crud in the oil, which seems very unlikely given your description of mileage/usage
Hard to imagine gasket issues.
All of which seems to be pointing towards sender or gauge issues.... Any chance of rodent attack on the wiring...?

Sorry not up to speed with the type of instrument fitted to later +8`s, there may be a simple way to check that there is oil pressure..? I suspect that with a 90`s +8 there might be a fuel pump cut out that depends upon oil pressure to allow the fuel pump to switch on.... Thus if the fuel pump was running long enough for you to discover the lack of oil pressure...? On the other hand perhaps the EFI system uses air flow sensing to allow the fuel pump to run...Does your dash have an oil pressure warning light...?

There are others better informed than I who will no doubt jump in when they read of your plight...