Check to see if the restrictor fitting is attached to the oil filter.It should have a small dia hole in it,as the oil filter is only working on the "bypass" system.You could try disconnecting the oil feed pipe to the filter with a suitable fitting,and if that brings your oil pressure up,you'll have something to investigate.
The other scenario is that the oil pressure relief valve is not seating properly.A mates '28 roadster had a similar problem.Pressure was 6-8 lbs.hot & driving,but barely 0-2 lbs at idle.I ended up stripping the relief valve,re machining the tapered valve seat,and making a new tapered valve head & fitting it to a new stem.After re-assembly of the spring and valve assembly,the pressure is now just over 15 lbs.(cold fast idle),and around the 8-12 lb mark driving.
