The oil pressure gauge on my 1995 Transport 3.8L was steady at about 60PSI when I purchased car new. After about 60K miles I noticed the pressure dipping slightly despite having the oil topped off. At 118K miles the engine was replaced with a rebuilt one. The gauge read 40PSI when driving, but dipped to about 30PSI when idle. Now, at 155K miles (37K on rebuilt engine) it runs at 40 but dips to 25 when idle. I have checked oil level constantly, and even right after an oil change it does this.
The rebuilt engine came with a new sensor and I assume a new or rebuilt oil pump. I know that the only parts removed from the old engine were the exhaust and intake manifolds. :confused:

Pressure is not level sensitive, in as much you have a decent amount of oil. However, mine ('93 3800) is very sensitive to the filter condition and to filter make: pressure always low with FRAM filters, higher with DELCO, and lowers with time.

First, 25 PSI at idle is fine, so don't worry. Oil pressure varies depending on base oil viscosity (10w30, 5w30, 20w50, etc.), oil temperature (cold start in winter vs. engine hot in summer), engine RPM, and the clearances in the engine pump and bearings (i.e., internal leakage). The oil pressure on your gauge is not really what matters, though. All the engine bearings need is a constant supply of oil, the motion of the bearings do the rest. This is because the oil pressure that seperates the bearing surfaces is developed by what is known to engineers as a hydrodynamic wedge. The hydrodynamic pressure depends on oil viscosity (oil grade and temperature), bearing clearances, RPM, and bearing load. As long as the bearings are getting supplied with oil at idle, you don't have anything to worry about. My Chevy truck (4.3L Vortec) idles at about 15-20 PSI with no problem. I don't necessarily like it (the more is better mentality, I guess), but I know my engine is doing itself no harm.