2000 CK1500 V8 5.3L Oil Pressure
 

My 2000 CK1500 V8 5.3L oil pressure gauge has been reading erratic lately. Now, when I turn off the engine, the oil pressure gauge reading drops partly the way to zero, but not all the way. If I immediately turn the key back on the the ignition position only, the gauge reading will fall to zero (or around there).

If this is a faulty oil pressure sending unit problem, does anyone know where the unit is located on this engine?

If this could be something else, I'd appreciate your comments.

Thank you.