I had a similar mystery pressure drop on my 8.1L...the pressure would still fluctuate across the RPM range was about half of what it used to read...at one point the pressure was down to around 15 psi at idle, but not low enough to show a low pressure message.
Issue turned out to be the oil filter. I had my shop do an oil change when it was in for some other work, and when it was time for an oil change again, I saw the filter had a huge dent in one side!! A new filter brought the oil pressure back to where it had always been.
Likely not your issue since you just did an oil change but thought I would share.