oil pressure on the 77 has been acting funny

[84 400]

So here is the scoop. when I start the truck the oil PSI is right around 40 or half way on the gauge. after driving for 30 min it dropps to just above 0. It still rises when accelerating but goes back down at idle. Is this something I should be worried about. I am not sure how much I trust the factory gauge. the engine is not burning or leaking any oil. I have a mechanical gauge on my other 350 in my 56 willys and it stays around 40 PSI all the time.

the idle is turned down on the motor and I can't rember if it dropped so much before I lowered the idle. My friend says his GMC Idles at 5 PSI when running for a while. As I said it does rise when acellerating to around 25 or so PSI. Could the sender be shot?? Or does it sound like my oil pump pickup or pump itself is going. The motor is bone stock with about 28,000 miles and has been changed regularly so I do not suspect too much sludge build up but I have never dropped the pan eather.

My 84 idles at 60PSI and rises off the gauge under acceleration but it is a built motor with a high volume pump and what not....

Any input would be great. Thanks every one!