Though the OP's problem could have many more possibilities, which I'm too tired to list, I wouldn't necessarily say valve train problems would be that much of a long shot, at least something to consider. I had an O-540 (Fresh Factory Reman, direct from Lycoming) that would not
achieve required oil pressure. I did not fly the airplane. I sent the engine back to Lycoming and they actually admitted they discovered an oil galley plug that was left out... I do need to at least give them credit for the admission rather than trying to BS me. Also tell maintenance you want to see the engine uncowled. Look for anything loose or out of place, especially on the induction side ( I believe it is normally aspirated?) Also look for rags/shop towels potentially left in the engine... (seen that happen more than once)