Correct Oil Filter for SB

A fishy pointed out that you have pressure to the valvetrain, so the pump is working. Oil comes up to the cam from the main bearings then to the rocker arms So you are getting some oil to the mains. Still sounds like a leaking or missing plug, rod bearings, or the rod side clearances. All can be checked by taking off the pan.
Since you have hydraulic lifters we know your lifter galleys are getting enough oil or your lifters would bleed down and tic.