Install a direct reading mechanical gauge. You can get the gauge & all the fittings from Home Depot for less than $15.
Next pull the distributor. here's a tech tip:
Take the coil wire out of the distributor. Pull #1 plug, put your finger in the spark plug hole. Have someone crank the engine until it starts to blow air past your finger. Stop cranking, turn the engine by hand (breaker bar on the harmonic balancer bolt, or put the car in gear and push) until the TDC mark on the harmonic balancer lines up with the pointer on the passenger side of the engine. Loose the distributor hold down bolt, remove the clamp, cap and wires, disconnect the wiring harness. Pull the distributor straight up.
Now that the distributor is out, use a 1/4 socket and a speeder handle or reversible drill to turn the oil pump shaft counterclockwise. You should see 20-60 PSI with cold oil depending on how fast you turn the oil pump shaft.
If you have good pressure, start troubleshooting the electric gauge.
If no oil pressure, plan on pulling the engine for a teardown. Bearings running without oil for more than a minute are definitely going to need replacement.
Follow the remaining steps to get the distributor reinstalled & timing set.
The following re-install depends on the engine being lined up as in the previous instructions.
To re-install, turn the distributor rotor to point to #1 cylinder position on the cap. The#1 position is about 11 o'clock if 6 o' clock is the front of the engine. Line the rotor up with it pointing at the #1 spark plug position on the cap and drop it in: you may have to twist the shaft some to get it to engage the oil pump shaft. Then remove the SPOUT connector plug, and use a timing light to set the timing. The SPOUT is the dangling connector on the distributor harness on stangs made prior to 94 and with a computer controlled engine. 10* is stock for most 5.0s' 12*-14* is good for more pep if you are not into NO2 or power adders