[iansoady]

1931 Model 10 owner here, so with its wet-sump engine I don't (can't) have this problem.

I don't think there would be a physical problem in fitting the filter. The return side of the pump should have excess capacity over the feed side and being a gear pump should have adequate pressure to overcome the filter's resistance.

However, I've never bothered with a filter on any of my "classic" bikes (apart from the Commando, which had one as standard). I have never subscribed to the view that detergent oil actually loosens sludge from the inside of the engine. What it does do is to keep any newly formed contamination in suspension rather than letting it drop out so that it leaves the engine at the next oil change.

[norman_john]

Hi,
On all the other non-vintage "classic" bikes I have owned, I have fitted a 'remote' filter on the return oil line - even on those with roller big ends.  I do this because I rather like using a modern quality detergent oil with all its advantages of modern additives and keeping the inside of the engine clean, added to which I rather like not re-circulating grit etc in my PnJ's oil. 

My 1930-ish Model 9 is my first "vintage" bike.  The oil return to the tank doesn't seem to indicate there is a massive flow from the sump - not that much air bubbling up at least.  So, my question is - has anyone fitted an in-line filter to a Sunbeam without it also having the effect of restricting the return flow to the extent that the sump fills up under normal running?  Or are the scavenge gears just too small...

Your thoughts and experiences would be appreciated!
John