Why oil pressure drop as an engine reaches operating temperature?

As the oil heats up it goes it flows much easier. This requires less pressure to move the oil throughout the engine. ALSO as described above, the viscosity (thickness) of the oil ,decreases which in turn makes oil flow, even in the narrowest area's, more freely. like electricity, more resistance, less current/flow, voltage (pressure) increases. but the reverse is also true..LESS resistance(thinner oil viscosity) when oil is hot. resistance to oil flow decreases, and the pressure (voltage) decreases.