Chevy 5.3 liter no oil pressure

Was driving my 08 silverado and the oil pressure dropped
suddenly to 0. I turned into the nearest place i could
(maybe 1000 ft). Turned the truck off and checked the oil. It
was low, so i put in 2 quarts. It read fine, so i jumped in,
fired it up, and the oil pressure was fine. I pulled out, made
it about 1/4 mile fown the road and same result, no oil
pressure. I pull over again and have it towed to the dealer.
They say its the oil sending unit gone bad,  i replace ti tge
tone of $740.50.......i leave from the dealer and make it
about 1 mile down the road and bam, no oil pressure again.
I pull over, call the dealer and am instructed to bring it back.
I do, now they're saying its not leaking, but oil is
disappersing and it still has 0 oil pressure and i need to
replace the motor....... what is going on?