Truck Repair: 93 chev z71 oil pressure, pressure gauge, plastigage

Question
Hey Van Thanks for the quick reply,
I have screwed on a pressure gauge if thats what you mean by mechanical gauge.The pressure eventually drops after 10 mins of driving then since there is no pressure i hear a soft knocking that will start but the truck runs and idels great. The knocking isnt there at first only when the pressure goes down.As for a sender i havent checked that how could i test that? Im thinking it's something real simple cause all the tuff things have been checked. Let me know what you think thanks alot.

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Followup To
Question -
I have a rebuilt 350 with about 50 000 km on it i changed the oil pump chcked the bearings it was all fine. But it didnt fix anything,The truck runs with great oil pressure for the first 10 mins but then seems like the oil heats up and gets thin and then i lose all the pressure. Can this be cause by the thermostat heating up the oil to much that it breaks down or maybe a hole in a line or somehting like that? If you have any ideas it would be great been a while with this probleme and no idea. Maybe gas is getting in and breaking down the oil if that makes sens. plz let me know what you think. Great thanks.
Answer -
Hi Jeff,
The gauge or sender could be the problem.
Screw in a mechanical gauge and see what it says.
Van

Answer
Hi Jeff,
The mechanical gauge was to test the accuracy of the regular gauge, and seems they all indicate the pressure dropping.
Dilution of the oil is a possibility, and would seem to act like that, but obviously you changed the oil...for sure when you dropped the pan.
Did you happen to check some bearings with plastigage to actually measure the clearance?
I really don't see the coolant getting hot enough to thin good oil that much...even boiling coolant shouldn't thin the oil enough to cause that.
I'm thinking maybe the crank was turned, and the correctly sized bearings were not used...maybe just on one journal even...and it could be a main bng.
If that is the case, though, it probably should have shown up shortly after the rebuild.
Is there some reason to believe fuel is getting into the oil?
You may have a candidate for some oil analysis.

Van