Chrysler 313 motor oil pressure issue.

20W60

I'd be putting in 10W30 mineral and seeing what happens. if cavitation is the problem, the heavier weight oil will contribute to cavitation.  ie; Thicker oil =  Higher flow resistance through the pipe and gauze filter. 

Becuase it happens so quickly and drops of quickly, I'm guessing that  it's just on the edge. The thinner oil might solve it.
Just buy some cheap stuff as an experiment. Drain the 20w60 oil into a clean pan so it can be reused if necessary.

Re:  " so i cannot see any connection between oil pump and throttle on or off at set revs whether it be 1000rpm or 2000rpm."   I've been assuming that there is an increase in revs.

Cliffo...............Just out of curiosities sake.......
What happens to the oil pressure on a long up hill drag where you can 
keep a full throttle situation for a while ..........
Does the lost 25 psi oil pressure ever recover or is it missing  while ever you have your 
foot down...........
............Billy..............

What are the chances that the Rowell pin on the drive gear of the oil pump has sheared, and at low revs the friction fit of the gear on the shaft is enough to give oil pressure, but slips once the revs are increased? Just a thought.

Be interesting to see what the problem is, are these pics any help?
  
 

 

What Grandad said.
While you have the sump off, clean the oil strainer on the pickup of the pump and check that Rowel thingo.

As a mechanic once told me, do the easiest things first.
 

Oil pressure stays at 30psi all the time your foot is down regardless of revs Idles at 30psi all the time. If the strainer was blocked the oil pressure would stay down I would think. Will check the measurement Grandad has given me

Thanks Grandad the measurement is spot on to the specs as in your reply so that rules that out

nothing is certain until the oil is at full operating temperature,then you get an idea of idle and full revs .pressure......Im used to Perkins 6/354 engines that have good oil pressure for several hours ,then it gradually drops to near zero.