Oil pressure gauge reads normal when first starts. as it warms up, gauge goes to zero. If I rev engine, gauge goes back to normal.

Two things...when it reads zero, is there any engine noise (tapping or knocking)? After you rev the engine does the pressure read normal at idle and function normally, or does it go back down? Either way, you should not drive the vehicle 'till you have a mechanical gauge (yours is electro-mechanical) put on there and the actual pressure is tested. The condition may be a bad sending unit, a bad bearing or possible obstructed oil pump screen. The repair is obviously different for each one and the price is as well.

2002 jeep grand cherokee with 4L v6...110k on motor...oil pressure reads normal at start-up and goes to very low when warm(with a slight ticking)....and goes back to normal when the engine is reved....ticking is NOT 100% constant however.....there are times of no audible engine noise out of ordinary....oil and filter just changed 200 miles ago...this vehicle was not properly mantained prior to my owning