Oil Pressure issue?

Good evening folks, my first post related to my first Duramax engine.
Information:

2007 2500HD with the duramax engine
6.6L engine says it on top of engine cover
Original 26,000 miles on it
On my way home I notice the oil pressure was low I think but don't know?
Cold start 50/55PSI
warm idle 30PSI
30-60mph 35PSI
60-70mph 35-40PSI
80mph 40PSI

Current history:
The oil needs to be changed, the truck sat for months at a time only used in summer to pull large camper.
Any advice related to proper oil pressure will be really appreciated. One last question, what brand oil and filter is a good one to stay with?
Thanks to all, Steve

sounds Ok to me...I use ACdelco filters and Amzoil..

Ive got an 06 been using Donaldson filters and CostCo oil.

217,000 miles, i have always used ac delco oil filters and delo 400 sde, isosyn advanced oil, sae 16w-40.