Since you are now not sure.......I will add another possibility......
The Dash Gauge Oil Pressure Sensor and I had one that all of a sudden showed lower oil pressure.....
after replacement oil pressure was fine (normal) at all Temperatures and RPMs
In general the NPT diameter of a plug is 1/4" larger than the NPT size.....
The oil pressure loss depends on where the "open" 1/4 NPT plug is located in the oil path. And depends on which oil path is involved.