Rick - My filter is a Bosch 3500. I've used other filters and still had the same phenomenon.
Also, I need to clarify that at hot idle (100 C, 900 rpm),
oil pressure is lower than at 2500 rpm (25 psi vs 45 psi). When hot, from idle up to about 2500 rpm, pressure goes up as engine speed increases, just like when it's cold. Then, when crusing at say 2500 rpm, a throttle increase causes a slight (2-3 psi) decrease in pressure, and a throttle decrease causes a slight increase in pressure. It's this that I'm trying to understand.
Thanks!