Thomas Neels

I have a 5.3ls motor. Blew the engine no oil pressure. I bought a used engine. New oil pan without the relief valve in it. new pick up tube with oring new high volume oil pump and new vlom. Still have no oil pressure to the top of the motor. Even using a mechanical Guage we finally got oil pressure at 3000rpm for 20psi and as soon as I let off the gas back to 0. What am I missing. I'm running 5w30 maybe a thicker oil or what's going on I have been messing with this issue for 3 months and by the way the screen whew the oil pressure sensor goes is new as well