Must've been happening a while: had 4 quarts in a 6-quart sump. Jeep never even hinted at low oil pressure, though, so that's a good thing. (When the oil pressure sender worked, which was 99.997% of the time, it reported correct pressure. When it wasn't working, 80 PSI. It hasn't done that for MONTHS though.)
The adaptor housing was probably already leaking, and I just made it worse by how I had to beat on the oil filter to get it off the truck. 203,000 miles--what do you expect?
Looked here and saw it's a set of O-rings:
Not very complicated, then, but still a pain. And it looks like the Fel-Pro set has the wrong main gasket, so I'll keep that in mind, too.
Welp--better order the gasket set and expect to do that job next weekend. Doesn't look like it'll take more than three or twelve hours. *sigh*