I had the timing belt changed at my local (trusted) mechanic and shortly after, my oil pressure went erratic. Sometimes working, sometimes showing low, sometimes not at all.
I was pretty sure it was the gauge since there was no noise/smells/temp issues, but I hooked up a manual gauge to prove the pressure was ok. Its fine.
I replaced the sender because it was the quickest and easiest thing and it worked ok for a few weeks but then quit again. I did some troubleshooting and it seemed to either be the gauge or the wiring.
Since I'd had the timing belt done and that moved the wiring, I took it back to my mechanic to have him look at it.
They diagnosed it as the gauge and had to order a (new) replacement instrument cluster from Toyota. I didn't want a used one with a different mileage, and the price they quoted me with labor was only a little more than the "certified rebuilt" ones I found on eBay, so I went with the dealer new one.
Its coming programmed for my truck with my mileage already done.
The test procedure, for anyone else having this problem, is on page BE-57 in the factory service manual.
The sending unit should have continuity when the engine is stopped between the pin and ground (case of the unit)
The sending unit should NOT have continuity when the engine is running.
To test the gauge, there are two pins on the back of the dash cluster which should have 25ohm resistance. If they don't, the gauge is bad.
Looking at the back of the cluster, it would be the top right pin right under the right most connector, and the bottom right pin which is just above the 2nd light in from the edge. There's a picture in the book.
Make sure you test the cluster off the car, nothing plugged into it.