Pressure drop on the Stock OE Gauge- not a Mechanical Gauge- correct? How low does the Oil Pressure “drop” at Idle on the OEM Gauge? How many RPM’s before pressure rises back up? It’s possible you may have a lower idle speed occurring as a result of an IAC, Fuel, intermittent Spark in a Cyl, etc..Check your idle speed on the Tach, if it seems any lower than what it typically has been (700-800 rpm’s is a rough guess) that may be what’s causing you’re issue. Start troubleshooting why from there- if Not- did this just start to occur one Day, or over a period of time, like a few Weeks to a Month or more?
If you haven’t already- Take the low Tech. approach.. A simple Oil/Filter Change with quality Materials, see where you are then. Can add other products to the Oil later- if needed..
I have been using both of these in My higher mileage Ponies from that Genre’ over the years- works great!.
Mobil 1 Super High Mileage Conventional Oil, or Synthetic- Mobil 1 High Mileage Synthetic, both @ 10-30 Viscosity. Complement with a good (Motorcraft, Mobil1, K&N) Filter... Good luck!