Mine does something similar. If it's idling, the stick is on 20. When I give it gas, it jumps back up to 40. Never been lower or higher, though.

I was always told it's normal.

 

oil pressure at 0 is not a good thing. was the motor rattling and complaining? 20 to 40 is ok. if your concerned put a mechanical oil pressure gauge on her and see what it is. imho dont use the cheap fram filters they do not like our cars,,,, or most any others.