For anyone who's interested:
The filter has a 140micron mesh. From what I can remember, most regular oil filters will advertise as capable of catching particles in the 5-30 micron range but in reality will pass an awful lot of material well over 40 microns. Don't get the wrong impression though - as a sole oil filter for engine oil this item would be a very bad choice - the mesh is reasonably coarse by modern filter standards and would not provide adequate protection.
As a rock catcher the 140 micron good enough for what I want - helping to prevent irreparable and immediate turbo bearing damage as a result of an engine failure. If I wanted to get more into it then I can drop the mesh size as long as it doesn't drop too much pressure but I can't really be bothered with that!
However - I'm not sure it's worth it now if either turbo feed is isolated all the way from the main filter....
I design stuff