Ever since the engine was rebuilt, I get quite a bit of soot in the silencers with a bit of oil deposit too. Some have said that it might be worn exhaust valve/guides?? I do use super-unleaded with an lead additive/octane boost. I don't always potter around and usually end the run with a 70mph blast up the M1 (10 miles or so). plugs are a bit dark with a bit of tan, maybe a bit of a rich mixture, but it ran brilliantly over the weekend (virtually no pinking up the hills, etc), and never missed a beat. Not overly concerned, change oil every 1000m give or take, what d'you sages reckon?