I Placed an Oil pressure Sending Unit from the JunkYard in My vehicle it showed Great pressure or Normal pressure at First Now it shows low pressure unless RPM's are up. last engine always showed Low pressure unless rpms was up as well.

What baffles me is it showed normal to High initially, are these known Faulty and is there a way to see The oil Pressure or check it other than a dash guage? New oil as its an engine from The junk Yard and just placed in truck.

 

sounds normal, the factory gauges are not really that great. As for the oil pressure with RPM, about every first gen I've ever owned has done that no matter the engine size.

 

You could always get a mechanical gauge and take readings at various RPM and compare those readings to your dash gauge. This way you will know what the varience is between the two gauges and you can mentally figure out what your actual pressure is no matter what the dash reading is.