As the above poster said, yes this is normal. In fact, this is normal for about all tj's and cars.
As oil heats up, it becomes less vicious and thins out., resulting in a drop in oil pressure at idle, when the engine isn't spinning as fast as it is when you are driving.
98 TJ sport 4.0L I6 356,000 miles.
No Engine, axle, tranny, or t-case rebuilds.
Check out my 5.3 swap in progress here!