Truck Repair: Oil pressure problem, oil pressure gauge, mechanical oil pressure gauge

Question
On my 86 Chevy blazer 4x4, I have good oil pressure at start-up 40 plus lbs. As motor warms up oil pressure gradually goes down and with in 3 to 5 minutes oil pressure is zero. I just put the motor back in for the third time, with new cam, barings, oil pump, oil filter, and sending unit. Thanks Martin  

Answer
Hello Martin,
I suppose you have already done this, but have you checked the oil pressure with a mechanical oil pressure gauge?
May be the gauge in the dash is wrong.

If you have replaced the cam bearings, surely you removed the heads and checked the head gaskets?

Oil isn't being thinned down with gasoline is it?

And then there are the main and rod bearings...as well as the possibility of a cracked head or block.

Van