Low Oil Pressure problem 2003 7.3

[Pelham]

I'm new to this so I hope I am working things correctly!
I have a 2003 4X4 4 door F250 7.3.. Bought it with 30K on it in 2005.. now it has 141K.. I recently learned the oil pressure gauge is nothing more than an "idiot" light, or "on/off" switch.. when it is at idle now it often, but not always, drops to the bottom.. I have had the actual pressure checked by a ford dealer and a local.. both confirmed hot idle drops to under 10 Lbs.. and reved up goes to about 30.. well below what it should be. I am told there could be 4 different possible problems.. the LPOP.. the Oil pressure regulator.. the by-pass valve.. or the engine is well worn!
I did change the oil, and put 15 W 40 Rotella T and a gallon of Lucas.. this did bring the oil pressure up some.
My question is.. if I am to approach a repair, where do I start.. is there a "most likely" problem of these 4 I have mentioned?? Or is there some other possible issue? I am installing a real pressure gauge soon.. I am also told that even though the oil pressure is quite low, as long as it does not go lower I should be OK driving it.. I sure would welcome some suggestions.. I am pretty much a total novice on this.. but my guess is maybe I should start by having the LPOP replaced and see what happens next
Thanks muchy for any response... Pelham

[CTC DEEZUL]

I would remove the oil pump(gerotor). Its on the front of the crank. Inspect it and the front cover. It should tell the a story. Its only about $80 new. Good place to start