The oil pressure depends on the speed of the oil pump, which is driven internally by your engine. When your engine is at a lower r.p.m., as when you are at a stop, the oil pump is working at a lower speed/volume, resulting in lower pressure. This low pressure is not a normal operating condition and could be caused by several different things. Is there adequate oil in the engine? The internal seals and/or rings may be worn and allowing oil to bypass them. Does it burn oil (blue/gray, smokey exhaust)? The oil pump itself may be on its way out. Excess trash in the oil due to infrequent changes can also lead to a build up of sludge and trash on the oil pump's pick-up screen, which results in low suction and oil volume at lower engine speeds. I hope that this helps.
