Can you explain these oil pressure readings?

We know that oil pressure is a function of oil viscosity, temp, and flow...basically.

So, if your bearings or pump have wear, oil pressure may be low. But, wouldn't it still be proportional to RPM?

Here are some readings:

At 1000 rpm, 1 Druck (15 psi or so)
2000 rpm 2 Druck
3000 rpm 3 Druck
4000 rpm 3 Druck
5000 rpm 3 Druck
etc...

Seems like at 3000 rpm the pressure peaks, and the flattens out... My first 911, a 76 with 120,000 miles did that...

Now, my 88 930 with 60,000 miles does also. At idle 1.5-2.0 Druck... Then it rises with RPM to about 3000 rpm then it flat lines at 3.5-4 Druck...

Now, I would think that bearing/pump problems would be proportional. IE, the peak pressure is lower than normal, but remains proportional to rpm...

What causes the plateau effect? Does that suggest a guage/sender issue? How can one have a great idle pressure, that shows this effect?. I have a mechanical guage on order.

When I recently changed the oil, castrol gtx 20/50, for the first 1 minute my idle pressure kept rising until it hit 4 Druck. Then it was like a valve opened, and it plummetted back to 1.5 Druck.

I replaced one of the relief valve springs (the one on the bottom of the case. No big change. I didn't replace the one on the side of the case (the emegency relief valve?).

Changing the sender made no difference...

So, what causes plateauing?