Extremely low oil pressure after oil change
 

I just purchased an 89 IROC with the 350 TPI. Vehicle is all stock with 77K. Previous owner was using Valvoline conventional 10W30. Vehicle had great oil pressure (30-60). I took it in for a full tune up an flushed out all oils replacing with synthetics. I used Royal Purple 5W30 with a NAPA Gold (Wix) oil filter. I took the car out for a cruise yesterday and the oil pressure kept dropping till it was finally at 12 to 14 pounds @ 2000RPM and ZERO at idle. I turned it off and let it sit for an hour. When I started back up, it was a little under 30 but kept dropping till it was again at zero when idling. Oil level was perfect on the dipstick. I'm not experiencing any pings, knocking or any other abnormal noises and the car runs extremely smooth and strong.
What seems to be the issue? Oil weight? Sending unit? Pump? Will Seafoam help? Any ideas?
I used the same oil and filter in my 87 Vette with the similar L98 and 80K on the odo with no problems. Any help would be appreciated. Thanks.

Probably the most effective cross-check would be to drain the Royal Purple and refill with the same oil that was used by the previous owner. If the pressure returns to normal, you have a reasonable answer. If it does not, you have already listed a logical series of possibilities to check. Begin with the simplest/cheapest and eliminate them in a logical sequence. Save the most complicated for the last, i.e., don't drop the pan until you've determined that the pressure sender is not the problem.