There isn't full pressure in the oil filter just like there isn't full pressure at the rocker arms..It is all volume at these points...The system is not as you think (sealed)...The pressure is being read off the tolerance areas..Aka..crankshaft...I'm guessing the bypass at the filter is about 15LBS..Fords don't have it in the block.That is why the spin ons to fit them have it there instead and they are about 15LBS...All engines need some sort of bypass because the oil filter can not handle the volume at higher RPMS..So....Only if the oil filter was like completely clogged would the pressure be affected...
