i just changed the oil on mine. i have an 94 with 5.2 in it.
has 220000 miles since today yay. never overhauled i believe and very very strong engine. i had kinda same pressure because the prev owner filled it with 5-30.
today i drained and flushed with one quart. then i put in 2 quarts of 10-40 then 1 engine treatment (lucas idea) and then 2 more quarts of 10-40.
i always use valvoline high mileage. and i use baldwin filter. so today did that started up and pressure is little bit higher then 40.
10-30 is what i use on my gf ford escort 4 cyl. so for my k3500 454 i used 10-40. after i did change i let it idle for like 5 or 6 min to get everything oiled up since i let it drain out completely.
My dad's old mechanic sometimes ran the wrong oil (cheap bastard...) and the needle would jump all over the place...i put high mileage oil of the stock weight (10w30) and it usually sits right at 40 at idle, a little higher under accel, and a little more higher at highway speed
JrMechanic's 100rpm/psi seems pretty true. You don't really need a whole lot of oil pressure to run an engine.
Factory gauges are a joke no matter who's car they're in. Many OEM's use oil pressure switches and a fixed resistor to feed the dummy gauge on the dashboard. It always reads midscale so the typical clueless driver is happy til it reads zero.
Not all oil filters are similar, or at least they didn't used to be. Some had check valve flappers, some didn't. The flapper helps keep oil up in the engine to help with dry starts.