Oil pressure is from back pressure.
The bearings, lifters and oil passages can only pass X amount of oil, the oil pump provides XXX amount of oil.
So all the oil can not be used and backs up in the oil passages where the oil pressure switch/sender is located, and that's the oil pressure in/of the engine.
When RPMs go up the oil pump pumps more oil but the oil passages flow doesn't increase that much so oil pressure goes up at higher RPMs and then down at lower RPMs
As an engine gets more miles the bearing gaps get larger so more oil can be passed thru, and back pressure drops down.
A thicker oil slows down the passing of the oil so oil pressure comes back up.
When engine is off the oil drains out of bearings and some passages, oil filters have a backflow preventer valve so main passages hold some oil.
But gravity is gravity so oil does drain out of some areas.
If your engine starts up quickly then you will often hear ticks from lack of oil pressure.
Oil treatments like Lucas aid in coating surfaces, doesn't drain off as fast, so it will often stop the start up ticks.
With my older high mile 4.0l I delay start up if it has been sitting more than 8 hours.
I press gas pedal to the floor and hold it down, this turns off fuel injectors
Then I crank engine for a few seconds to pump in some oil
Then I release gas pedal and start the engine
I run 10w40, have for a few years now