Does anyone have any idea why the oil pressure in a 454 appears normal until it reaches operating temp(190)but then drops to near zero?

With straight 30w, my 454's oil pressure needle pegs out until operating temp, then runs about one needle width lower during highway speeds......If you're satisfied that oil quantity & viscosity are OK, I'd suspect gauge sending unit as a first look.
Oil pumps are pretty uncomplicated machines, and tend to either totally work or totally fail. I doubt that what your reading on the gauge is reality, but it sure can be unnerving......

My 81 w/89000 miles pegs to 60 then gradually declines over a few hundred miles to 40 or so, but stopping and idling drops it to scary after that long a run. Never had a noise or temp problem so I just drive it. I use 10w30 in Florida but I think next time I'll go straight 30 or 40.

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Dan & Suzy Z
'81 Euro 28

Thanks guys. I'm thinking the gauge is the problem. At startup and when I rev the engine the needle pegs all the way to the right then jumps eradically all over the dial.

You might want to check the wire attached to the sending unit on the head. If the wire is loose, it can cause the gauge to peg out and do crazy things. I had the same problem.