Low oil pressure

[Bigcassells]

I have a 94 chevy 1500 z71 it has the 5.7 350 vortec in it. It only has 76000 miles 2 owners including me. The first owner died and the truck sat in his estate for 5 years. I bought it from a reputable used car lot in Albemarle nc.
I drove it on the test drive and noticed it had lower oil pressure than my 93 gmc 1500 so i thought no biggie its the nature of the beast...
Well as time went on it got worse the oil pressure droped more and more.
First i went with a new sending unit.
Then i tried a thicker oil. Used 20w50 which is what i run in my 93 because of the wear on it.
That didn't help so i had a friend replace the oil pump/pick up screen for me. Because of work i couldnt do it myself.
then we installed a mechanical oil pressure guage and it was showing the same.
It had enough oil pressure to run the engine but running 55mph at 2200 rpm it was only reading maybe 15 psi oil pressure.
So now i have replaced the engine as a last effort to fix it and the oil pressure is still low at idle after the engine is warmed up. When you put it in drive with you foot on the brake it drops a touch as normal and when you leave the stop sign it comes right up to just under 40 psi. And the engine was a brand new( not remand) long block from chevrolet. Im running 5w30 as it calls for. Im just fed up with the gauge not reading where it should. Any ideas? Thanks.
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