The V8 Owners Forum -

DEVONMAN wrote:

DaviesDJ wrote:There is no rubber non return visible on the k&n and halfords filters I have - but none visible on the britpart

Filters may have a non return mechanism or a bypass valve or both.

The bypass valve is visible at the bottom if you look down through the centre hole of the filter.
The non return mechanism takes the form of a rubber disc which is positioned inside the filter can behind the perimeter inlet holes. If you can pour oil straight through these holes then there is no non return valve fitted. The rubber disc should appear to be blocking these perimeter holes.

Without the non return mechanism their will be a possibility that the oil galleries will drain down towards the sump and in some cases the sump pickup pipe will syphon oil out of the pump/filter and cause delay to pressure build on start up and in unusual/rare cases, the pump may not automatically self prime.

The bypass valve will open when the filter is clogged so as to allow sufficent oil to reach the engine. However some crud parked in the filter can will also get through. The valve can also open if the engine is over reved from cold when the oil is thick.