Oil pressure reads 20 with just accessories on, 60-75psi, while driving , what is problem?

These are normal oil pressures. No problem. If you start to see oil pressures under 20 lbs, there is a screen under the oil pressure sending unit that gets gummed up, causing a false oil pressure reading, may even shut the engine down. At that point it is just a matter of pulling the oil pressure sending unit, and fishing out the screen from the hole. Either clean it or replace it.

I replaced o.p. sender after oil pressure dropped to zero, now o.p. Fluctuates with rpm, a lot more than it used to. 50-65, Any ideas?

Clean that screen I mentioned.