The pressure port in the top of an FE
oil filter adapter is downstream of the filter element and cooler. It connects to the passage which sends the
oil to the motor, after it has already been thru the filter and cooler. When you connect your guage to that port, you are getting the pressure after the filter, after the cooler. Your pressure sensor is the last thing the
oil sees before entering the block. The reason why Joe saw an increase is the difference between Fram and wix filters, many others can confirm the same results.
So, in short, you are getting a "true" reading of the pressure as it enters your block.