Okay, here's what's happened:

I managed to get the oil pressure relief valve out, and everything was
there.  As a matter of fact the valve just fell out with the spring,
washer, and cap.  So, I'm not so sure the relief valve was at fault, but I
did clean it up and stick it back in.

I did put in a NAPA oil filter, and I was told this might be the problem.
So, I'll definitely switch oil filters at the next possible chance.

I don't know what oil was in there before.  I happened to have Mobil 1
10W-40 sitting around from my H-nda(sorry!).  I'll switch back to regular
motor oil.

I'm not sure if the connecting rod bolts were reused during the rebuild
which occurred 2 years ago.  I'll call the mechanic and find out.

Thanks to everyone who responded, and I'll make sure to post an update
after I've done all of these things.

-Mike