"Things" do not "open up" when the engine gets warmed to operating temperature. Oil pumps are rarely the issue either, although the Buick V6 did have a bit of a problem with the outside mounted pumps. The oil path was convoluted.
The pressure drop is caused by the oil thinning out as it gets well warmed up.
I repaired a number of Ford pickups with 400 engines. The Clevelland design engines are known for not having the best lubrication systems, and the 400 was based on the Clevelland. At about 100k miles the OP indicator light would start to flicker when stopped in Drive at red lights or stop signs. The farm boys would come in and ask if I could install a new oil pump. I would pull the oil pan and clean and inspect the oil pump, and never found much for wear. Witness marks, yes. Check the main and rod bearings and they would show copper on the top and bottom shells. I then checked the timing chain which with that mileage was worn. I then phoned and explained what I found and a price for replacing the bearings and timing set. Always installed a double roller timing chain. I also installed 0.001"US bearings to take make up for the journal wear. This usually brought the clearances close to the minimum spec. A couple of guys were upset I did not replace the oil pump. I told them to drive it for a week or two and if they were not happy, I would do the labor at no cost if they paid for the pump. None came back for a new pump.
Bearing clearance is a "leak". When the oil is cold it does not leak out the clearances as readily, so oil pressure looks fairly good. As the oil gets well warmed up, it leaks out easier, causing a pressure drop. If the clearances are on the high side with warm oil, more leaks out and the pressure drops. As you rev it up the pump pushes a lot more oil volume, so the pressure improves.