The oil pressure gauge should indicate a substantial amount of pressure after a cold start. Think about it. When oil is cold, it's still relatively thick requiring more effort to pump it through all of the lubrication journals and therefore registering increased pressure.
As the oil heats up, it thins out (basic thermodynamics) and is therefore easier to "move" through the engine. Thus, less effort and in turn pressure is required.
I deally you'll want the gauge to be at 3 all of the time. However, if it shows 1.5-2 at idle while warm, that is perfectly acceptable.
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