Back in the day when I used to run with a felt oil filter I used to check them when doing an oil change, there was always sparkly little pieces of metal embedded in the felt, hardly surprising when on a rebuild it was found that the main bearing had walked and the crankpin nut was grinding itself away on the bearing outer race, a very common occurrence I believe. That fault was corrected (hope it still is) and I now use a club spares style paper filter, it does concern a little whether the rubber seals at each end actually make an oil tight seal on the sand cast aluminium at the back end of the filter housing, but whether it does or not I would always use a filter, and a magnetic sump plug. Phil Irving saw fit to design the engine to run with a filter, and I bow to his knowledge. I own an early Honda motorcycle, a CB77, a very well mechanically designed and manufactured early Honda, when I first purchased the bike in 1999 one of the first things I did was to change the oil and check the filter housing, it does not have a conventional strainer type filter, but runs a high speed rotating cylinder to centrifuge the debris to the internal wall of the cylinder, with the centrifuged clean oil flowing out through a central orifice, well I can tell you the filter cylinder was heavily encrusted with loads of hard, solid debris, probably never been cleaned and only running at a much reduced filtration capacity, this is why we have oil filters, and why we should change them regularly. Obviously Bill has loads of experience and knowledge, and I bow to his achievements, but on this one I think he is wrong, run a filter, you know it makes sense.