2011 Chevy Silverado low oil pressure problems

Truck has 125000 miles on it and had pistons
and rings replaced at 55,000. Recently got
my truck back from the body shop. It ran just
fine when I brought it there to be repaired but
since getting it back I have low oil pressure
problems. I got 20 minutes from the shop
and I noticed I slowly lot pressure to the point
it told me to kill the engine. I stopped it,
thought about towing it from that location but
for the heck of it started it up and I had 30
pounds pressure again on the gauge. Drove it
the last 2 minutes home. No check engine
light ever came on. Replaced the oil sending
unit, and filter below it. Filter looked fairly
clean but replaced it anyway. I had almost 40
pounds pressure after that. I drive out of
shop, made it a mile down the road and
check engine light comes on and pressure
slowly dropped to the point it told me to kill
the engine. Find out it’s a P0521 code.
Engine Oil Pressure sensor Range/
Performance. Thought I got a faulty sending
unit. Replaced it again.  Check engine light
come on again. Pressure is currently running
between 20-30 pounds. Oil change has
about 1000 miles on it.