Moto Grand Prix
Main Motorcycle: T110 Triton
Join Date: Nov 2012
Location: Darkest Pulteneytown
I'm something of a parvenu, so I may have the wrong end of the stick.
Putting a filter or any potential restriction in the line from the oil tank to the feed side of the pump is bad practice. The Charlie's filter in the bottom of the frame seems to give good service, even though it is theoretically bad practice, but it's different, sitting there immersed under the head of the oil level. The usual place to put an aftermarket filter on old dry sump bikes is in the scavenge line. On a T140, it is easier to fit a Charlie's, so many owners do that.
Plumbing a separate filter, such as a Commando one, into the feed line is a bit dodgier. Any air leaks at the extra hose joints or the filter O-ring could mean the pump draws air instead of oil. A dirty filter building up resistance to flow could cause the hose to collapse under vacuum. The thing is, even if you see these risks as small, you're getting no advantage. You may theorise that it's a "better" place to filter the oil, but it won't improve your riding experience, compared to filtering the scavenge.
Putting the filter between the oil feed pump and the relief valve is theoretically not bad, although it still leaves the pump at the mercy of any dirt in the oil. It has to introduce a fair bit of extra plumbing, all at full oil pressure. If you get a leak there, it'll be a big one.