First gauges are inaccurate! Verify the actual numbers with a mechanical gauge.
IMHO That is a thin oil for that engine. You might go to a heavier weight to see if that keeps the pressure a bit higher when warm.
Most likely you have worn main bearings-at least to some extent and the oil pressure is dropping because the oil has excessive space to travel through and can't create back pressure.
if something in the neighborhood of a 15W40 can't keep your pressure at a true15 pounds at idle, you are likely looking at a rebuild.
rule of thumb is a minimum of 10 PSI for every 1000 rpm the engine turns.
so at 2500 rpm, you really should have 25 psi at a minimum and at 5500 rpm 50psi is only okay.
If pressure was good and this is a new development, it could be a sensor or a gauge, but it could also be spun main bearings.
I spun the main bearings in a boat motor last summer and heard a change in the engine note under full throttle before the gauge even registered the drop in oil pressure. I couldn't explain what I heard to anyone else in the boat, just that it had changed and sure enough I throttled down and the oil pressure dropped to virtually nothing.
I think you just have some wear and too thin of an oil. 5W-30 is designed for modern high tolerance engines. A 10W-30 or 10-40 or 15-40 will be more appropriate for old iron v-8's, especially with some miles on them.