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Check the oil pressure regulator. It screws into the front timing cover on the passenger side near the cam sensor. check to see if it is stuck .
I don't know why. But it is located in the front timing cover. It takes longer to get to remove the fan and bracket over it than it does to remove and inspect it. You might be able to get to it from underneath also.
Here is an exploded view of the internal oil pump. Shows no oil relieve valve.
Here is an exploded view of the front engine components. The oil pressure relieve valve is #55
Here is a close up of the timing cover. The relieve valve is on the rt side of the cover near the crank sensor openings. It shows the hex head on the end of the valve.
So there is where it is located. I can only assume that the dealership employee that told you that is not very knowlegedable on the duramax.