The oil filter was a regular Fram PH13. I did not cut it open, only removed it and poured the oil out. The oil in the filter was the same color as in the pan: bronze-ish with no visible metal shavings.
The oil galley plugs at the front were replaced by a previous owner of the engine. They looked fine and intact so I left them as they were.
Any other ideas about the cause for this type of sudden pressure loss? Like I said, the pressure was fine at first, on Saturday (the initial first start) and today. The pressure dropped from ~60 psi to 20 psi while accelerating on 2nd gear to 4500 rpm with the engine warmed up. After that the pressure rises to 40 psi at idle but doesn't go up with revs.