The story was his wife was only allowing him $12k to spend on a plane. He had retired and sold off a Tri-Pacer and missed flying. He seemed ok to me. Maybe his plane was to buy a nice, but high time airplane for $12K. Then ooops! Sorry dear, it needs an engine.
Sorry no checks, especially for more than the amount of the sale. I might have been born at night, but it wasn't last night.
I'm not sure about the oil pressure before the filter adapter was installed. It was put in 2001. Then the plane was then sold to the previous 2 partners who owned it and moved it to FIT. The screen was never removed or cleaned again until a couple months ago. Evidently the mechanics never bothered to look.

Following their lead, neither did I.

The last owners used the same mechanics as I do.
Is there any chance that the oil filter adapter is causing the problem?

I wouldn't think so, but it isn't out of spec by much.