[Lawrie]

Pooch the bypass valve in the oil filter only measures the differential pressure ,that is the difference between the oil going in and the oil coming out of the filter,that is called the pressure drop over the filter.
On my A it is about 3.5 psi hot at 40mph.
I use the oil filter from a 6cyl falcon on my v8,s as they have the bypass valve in them,
the valve is there to still allow oil to get to the engine if the filter is not changed and becomes clogged or the engine is reved up with cold oil.
Lawrie