I have been pondering extra (magnetic)filtration on the twin for a while. My take on it is thus: The stop valve fitting at the oil tank outlet could be discarded and a slightly bigger diameter unit made, with magnets fitted inside to arrest any (ferrous) metallic contamination of the oil before it gets down to the motor. The standard gauze in the oil tank would have to be left as is i.e. no finer, as there is only gravity feeding the oil and obviously no restriction to free flow can be allowed-( not that it is generous anyway- the standard outlet fitting under the stop valve is little over 6mm dia!!) I think a second filter unit would be best placed in the scavenge return BEFORE it feeds the rockers- no point in picking up the engines natural wear particles just to put some of them back in again via the rocker feeds. Not so easy to site this one due to lack of space- although a crude method would be to stick magnets to the outside of the A58AS and A66AS pipes to arrest the metallic crud onto the inside of the pipe-not easy to check or clean....If a reliable filter could be fitted in the A58AS area (bearing in mind that here this is pumped oil ) it would perhaps obviate the need for a filter on the entry back to the tank, unless consideration is to be made for non ferrous contamination also, where we have to move to a paper/fine gauze element.There are some very fine and compact gauze element oil filters made for small capacity motorcycle engines "HI-FLO" i think is one make.Placed in a suitable custom made pot, perhaps with a magnet in there also, and squeezed into the gap between the cylinders under the oil tank return port.I would think a suitable size could be found to cope easily with the flow OK, thus trapping any appreciable crud before it gets dumped in the oil tank. Discuss/ridicule as you feel appropriate..... Bob.