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Q:

loosing oil pressure on 1998 Chevrolet 2500 Pickup

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
had a 5.7 motor blew replaced it with 5.0 crate motor nothing leaks everthing look fine changed 10 of these never had a problem.normal at startup but falls quickly changed oil pump.
(1) Answer
Do the oil pans and pick up tubes match with the 5.7 and 5.0? Are you verifying the loss of oil pressure with a manual oil testing gauge or are you using the dash gauge which could be off or the sending unit is bad etc? These are GM crate motors?