Barrel just fitted and rings seem to make an even sweep of oil residue, no streaks, so it can only improve with time.....hopefully. I notice that the new rings appear to be cast as opposed to the previous set which I Believe were steel. That could make a difference as perhaps the previous set were not bedding in.
I liked Nev's suggestion of an 'O' ring fitted below the retaining cap, but the inner ring is a snug fit on the cap so no provision for encapsulating a washer. Research shows that Minis and MGBs got by with just an 'O' ring allowed to slide on the valve stem. I will try doing the same with Viton rings as anything else would not cope with the temperature. Just a belt and braces approach and may mean not having to dismantle the old girl again. It won't hurt . However, I believe rings were the problem and not the new valves and guides so time will tell.
1960 Matchless G12
1958 AJS 18s