Oil filter on oil return

[shaunstaples]

I finally got round to putting a pressure gauge on both sides of the oil filter and the output is approximately 50% of the inlet pressure when the oil is hot or cold. When cold the input was easily going over 20psi with the outlet around 10psi. The filter spec says 18psi bypass pressure but think I can see the inlet pressure gauge fluctuating in a pattern as if the bypass valve is opening and closing, or the return pump is clearing and drawing air but don't see it when the oil is warm

When oil is cold


When oil is warm

[shaunstaples]

Thinking again…
Is it just as simple as a restriction in the outlet of the filter that balances the in and out of the filter ( similar to the ones in the top end feed ) then the pressure difference will be only from the blocking of the filter.
Oil filters are on the feed to the crank normally, so the pressure is governed by the resistance of getting oil through it.
I could try a tap in the pipe from filter to tank, that could vary the flow to test it?
No substitute for a dirty filter on your service, not getting any value for money if not!