Oil Pressure Sensor / Sending Unit
I couldn't find any simple way to test this sensor other than by replacing it. My pressure gauge would pin at maximum when I first fired up the engine, but after about 20 or 30 kilometers, the engine would warm up and it would start jumping around all over the place, so I took a chance and just replaced it. And yes, replacing it fixed the issue. With the new sensor installed, the gauge now always sits at about 40 (half way on the gauge) with the engine running.
To get an idea of where the Oil Pressure Sensor is located, it is on the back of the engine, on the driver's side below the distributor. To get access to it you will need to remove the entire air cleaner housing.
I didn't take a picture of the original while it was installed, but here are two shots of the hole where the sensor threads into the engine. You can see how difficult it is to get at.
To remove the original sending unit (shown on the left), I had to track down a 1 3/16th inch Oil Pressure Sensor Socket. A regular socket isn't deep enough to reach the bottom of the sensor, and there isn't enough room around it to use a wrench.
The next problem was, the previous owner had installed a standoff on the bottom of the sending unit with about a twenty degree offset. The standoff was necessary, but the offset wasn't. To make removal even more difficult, whoever installed it used a thread sealer that made removal of the sensor and standoff impossible without breaking the standoff. And that was exactly what happened. 		Fortunately, while trying to remove the sensor I had loosened the threads of the standoff, and I was able to remove it with a No3 Screw Extractor (Easy Out) and a long extension.
Here is the new standoff in place. Notice I used a straight one without any offset. Again, the offset isn't needed.
The reason the standoff is needed is because the protrusion on the head that I marked with a red dot will prevent the threaded part of the sensor from reaching the hole it needs to thread into.		 Here is the new sending unit installed.

To install the new oil pressure sending unit, I had to get a different size Oil Pressure Sensor Socket. The new sending unit required a 1 1/16th socket.