oil pressure question

It's probably worn - bearings and the pump itself. It's also probably fine. If you're nervous, buy a new pump, disassemble and clean it and make sure the bypass works, and install it. My impression is it might help a little in terms of your confidence. But that's about it.
And yes - lose the STP. Chances are the 20 year old camshaft will be just fine with any racing oil, or the fleet 15-40.