I was surfing around trying to get specs on oil filters and found some surprising info on the WIX site.
The Ford filters have a relief or bypass valve set between 8 and 11 psi. That would suggest (taking flow out of the equation) that an average engine with an oil pressure of 30 to 60 psi would pop the relief almost all the time even at idle and only filter part of the oil. Obviously that has worked well for many years but I was surprised at how low the valve was set. Any comments?

 

That is pressure inside the filter. if the restriction is in the filter media than pressure builds enough to bypass. I assume that a normal, clean oil filter only takes a few psi to push it thru the filter.