Btiesinga

Hi Everyone, New to this forum, I was hoping I could get some input on an oil pressure problem I'm experiencing. I have a 2004 2500HD 6.0 Vortec with ~84,000 Miles. Recently I've had an issue where once my truck is warm the oil pressure steadily drops at idle. It will start at about 38 PSI and after 10 - 15 minutes it will drop down to roughly 5 PSI. I confirmed with a mechanical gauge. The same issue occurs when I drive the truck for a similar amount of time. Also Upon startup, I get a "CHECK OIL LEVEL" message which goes away after about 3 seconds. The oil level is up to where it should be on the dipstick. Here's some of the things I've done that HAVE NOT worked along with some of the work I've hired my mechanic to do, all within the past 4 months. I also put some pictures in for reference. - Replaced oil Sending Unit (No filter on Models before 2007) - Checked to ensure dash gauge was not broken - Replaced Oil Filter - Replaced upper and Lower Hose - Replaced oil pump - Replace Oil pick up tube - Installed new oil pan (Lots of build up, as pictured) - Replaced oil level sensor. I'm fairly new to this, but I'm stating to think it is a engine issue. If anyone has any suggestions for what I should try next or any other input I'd really appreciate it.