9 psi difference in oil filters!

[SS Tim]

While your observation is interesting it really needs to be tested to verify the actual change. It sounds like one filter reading was taken from a coldsoak and overnight drain back while the other was on a warmer engine that had not sat overnight.

If you really want to know, go buy two new filters of the brand and model you want. Start and run the engine or drive until fully warm. Note the current idle oil pressure.

Now without delay replace the filter with a prefilled test filter #1. Start and run the engine again back to operating temp., note the pressure indication.

Imediately remove the #1 test filter and do the same with test filter #2.

Remove the #2 test filter and reinstall the #1 test filter again prefilled and note the indications, they must be the same as the first test run for a valid test.

Anything less than this is an observation and not a test.