oil gauge not working properly

change the oil sending unit.

If it's not the gauge or sending unit, your oil pressure is dropping because the oil sump is high mileage and worn/clogged and can't pump the oil at such a low idle after the viscosity has changed when the engine is warm. Its not a problem I'd bother with but if you really want to fix it i would recommend dropping the oil pan and replacing the pump and pickup, or at least check if the screen on the bottom is clogged with debris.