Once again Sluggie, I am not cranky,
And I do concur with points 1 & 2
Short version:
#1) A filter with modern oil in vintage bikes is a good idea
#2) Use a filter with no restrictions, check valves, anti reversion flappers or anything else. WHY? Because in the end there is NO benefit from those on old BI, NONE,, BUT it CAN, not always but CAN be a source of problems, So, Murphys law would dictate why introduce a potential problem for no reason?
However the rest is just clouding the issues and only just barely relevent , unless you are as you mentioned trying to pull 300% more out of the engine.
But the OP is not doing this, he is simply asking if he should fit the filter, where should he put it & what filter to use.
Now considering the substantial extra cost of very fine oil filters , extra cost of clogged filter by pass valves , let alone reverse flow check valves, I seriously doubt this would be a problem.
Finding a cheaper filter elements seems to be tops on most agendas.
Now perhaps Notrun owners are happier to open their wallets than BSA riders are but the only question I get regularly is "where can I get a cheaper cartridge from ?" closely followed by "is there a smaller filter element that will fit?"
And again, max flow rates and the like are well & good but totally irrelevent to what we are riding.
Considering how well a lot of BSA's run with the sludge trap all but blocked off, flow rates are a bit etherial.
Right now there are a couple of photos on this forum of choked up cranks from bikes that were running quite well.
There was a reason why BSA recommended 30,000 between bottom end jobs and it was not because the slippers would be worn down to the backing metal by then.
Using the high ash oils of the day at 30,000 the sludge trap can be getting down to 1/4 volume.
If you have done any where near as much work on BSA bottom ends as you mention then you must have come across more than 1 where the oil gallery was blocked solid & the oil was more like seeping through a solid mass than flowing through a gallery.
Julian got it right in post 47, way too much over thinking.
Having a wealth of experience and the knowledge to back it up is a good thing and spreading that knowledge is admirable, in the right place , at the right time.
You have mentioned a lot of very interesting things, and in their own thread would be fantastic but none of them was any help to Rowan.
Mind you I can also run way off topic as well, you are not alone.
As for the remains of the in tank filter Rowan posted, they slip over the return line standpipe in pre-WWII Std & Deluxe BSA's.
They were not fitted to competition models.
If you look down the standpipe on an oil tank that had one fitted you will see a base plate on the bottom of the standpipe.
No bottom plate = no filter.
While the filters are being remade in Tiwain now days the top cover is not and without the cap which forces the oil down through the felt the filter is of no real value.
If you are buying a competition Blue Star it will not have one as because as you should know it loads up the return side of the pump which robs power from the crank and the Blues Stars were the performance models in 35, 36 & 37 . BSA fitted 7 different engines to them and naturally the semi wet sumped models had no filter, in fact there was no return pump at all , more horses for the track.