back when my exaust vales were burnt. I'd run 60psi plus @3000 for about week, then it would drop off as my oil went down to about half, I'd adda quart and it would rise back up. It did this reliably for a few months, and I was VERY anal about watching the pressure gauge and keeping track of any changes whether due to temp/ run time/ oil level. Maybe it had something to do with the oil "breaking down" due to the exhaust heat coming in through the valves and lowering the viscosity/ thickness= thus lowering the oil pressure.
All I know is that my oil P would drop with the oil level