Is my oil pressure dropping too low when hot (1992 K1500)?

The oil pressure gauge on my dash looks fine when I start the truck but it starts to drop after about 20 minutes of driving. It was getting to the point where the warning light would come on when idling. I got a mechanical gauge and hooked it up to the engine. When it’s cold, it reads about 30psi at idle. After warming up, it will stay between 15-20psi while driving around 2k rpms but drops down to 5-10psi when idling around 1200 rpms. Is that getting too low? What could cause this?