oil pressure drop when hot

[Shon]

Coming to you guys for some advise. I have not had this issue come up before LOL

Junk yard 6.0 from 2001 2500HD Silverado, mileage unknown, assumed LOTS.
replaced lifters, chain, and oil pump with new GM units.

Idle 40+PSI, rev it up get 60PSI. So I didnt think/notice any issues till I started driving it. Then noticed about 15-20min in the drive oil pressure is starting to drop. Just enough time for the oil to actually get heat in it and thin out (is my assumption). It drops to 20 at idle (or lower sometimes) and will not build past 30PSI.

F-body pan with Fbody diptstick/tube. oil is full. 5w30 non synthetic oil.

Thought if the o-ring was not seated right OP would not build at all.
I have read that some Oil pumps are bad out the box, not sure how b/c theres not much to them but it happens.

I am open to any and all suggestions before pulling front cover off and replacing the pump. Most research says that is the correct action plan. I do have the old pump here that can be used for a test.

Mechanical gauge.
All ears!