I have been running an 1800 8V conversion in my '82 westy for almost 5
years now (approx 65K miles/105K kilometers), and I have tried a few
different filters:

Fram HP(HD-1?)-1
VW I-4 diesel engine filter
VW I-4 gas engine filter
VW wasserboxer filter

All of them, except the wasserboxer version, would allow some of the oil to
drain(siphon?) out of them overnight, resulting in a 3-5 second delay of
oil pressure build up upon starting (next morning). This the length delay
was NOT consistant, infact it didn't always occur.

After a discussion on this list a short time ago I decided to try the
wasserboxer filter and I have found that my oil pressure builds up almost
immediately upon morning starts.

The obvious difference between the I-4 and the wbx filters is location of
what I can only assume is the anti- backflow valve. On the I-4 filter this
valve is close to the mounting base, on the wbx filter it is further down
the main oil passage, distal to the mounting base.

Oh, BTW, my oil pressure sender is mounted on the cyl head.

Alistair