Low oil pressure is not a problem that can solely be attributed to a problem with the oil pump. It could be that the bearings in your engine are worn out and the engine is in need of a complete rebuild. It could also be that the gauge is inaccurate.
An external oil leak will not affect pressure.
I personally do not believe in aftermarket oil treatments as I am sure almost everyone here will agree. I don;t think there is even a snake oil that attempts to make the false claim that it can raise your oil pressure.
Oil pressure problems will not show up with diagnostics.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record