Low Oil Pressure

I have a 1996 Chevy K1500 with a 5.7 motor
we just put a new motor(but used with 158k
miles) in it replacing the oil pump brand new.
We noticed that has very low oil pressure. We
thought make the sending unit so we took
that out and replaced it with aftermarket oil
gage still shows the same thing. Idle, park, or
at light stop sign/light it have 5-0 psi oil
pressure showing, driving too speed of 65 it
show no more pressure then 20 psi. There no
noises coming from motor no knocking,
nothing. Runs and sounds great! But only
problem we’re seeing is it shows very low to
no oil pressure.. any suggestions or advice?
We tried 10w-30 motor oil no difference so
we go back to original 5w-30 motor oil and
no difference...