I assume 1.5 bar or about 22 psi is the point above which the oil
pressure causes the bypass valve to begin allowing part of the oil to
continue unfiltered.  If there were no bypass valve something would have
to give... maybe the filter media and/or a critical lubing function of
the oil in the engine may not be adequately carried out.  I guess it
might be important to protect the integrity of the filter or to insure
sufficient oil pressure to the bearings.
-Dana-

> Do you suppose that the 1.5 Bar rating indicates the pressure differential
> between upstream and downstream of the element? Or is it a rating referring
> to normal atmospheric pressure?
>
> It seems that if it is the latter, it would be in bypass mode during most of
> the duty cycle of the engine - and that would not seem to make sense.
>
> My Van appears to be equipped with one of these valves integral to the
> filter mounting stand - and it seems to be of the differential design.
>
> Tom and Dana Cates