If you don't hear valve train "ticking" noises then oil pressure is OK.
Probably a dirty or bad oil pressure sender.
In the mid-1980s Ford switched to an on/off type oil pressure gauge/sender.
When engine starts and oil pressure is above 6psi the sender closes(on) and passes the engine ground to the dash board gauge, the gauge has a resistor that will cause the needle to go to about 1/3-1/2, and stay there, if it moves at all it is because of voltage not oil pressure, just FYI.
If sender sees below 6psi then it stays open(off) so gauge stays at 0
If the sender opening or passage gets sludge in it then it may not close(on) all the time, oil may have to get thinner(warmer) to close the sender.
Oil pressure senders are usually located near the oil filter, not all the time just usually, they will have just 1 wire connected.
Remove and replace if you want or remove and clean.
Also, with sender removed unplug coil, you want a no start, and crank engine, just a second or two, have something under engine to catch any oil, this will clean out the senders passage.
DO NOT use sealing tape on a sender, the threads are the Ground needed to complete the circuit, so always leave a few threads at the bottom bare to get a good ground to the engine.