I own a 1991 1500 (manual). A few years back the motor blew and it got replaced from by a motor with an '89 1500 (350 motor, original was 305). Ever since the replacement, when I'm at idle, it shows no very little to no oil pressure, and if I'm driving and put it in neutral, the oil pressure drops, engine bogs down, then it recovers and goes to idle. Any ideas? Thank you in advance

Edit: Forgot to put; it only does this after it's warmed up