Same behavior with my 1971 Fastback....Every Norton person I have discussed this with to date says the "breather" in fact is a "spitter"...general consensus also seems to be the "L" marking is the beginning of the "normal" oil level, the mid point between "L" and "H" is the highpoint, and over the mid point is over filled, and will increase the spitting....Speaking about the breather, when I got the bike back after being restored it had a 1/2" breather line running from the front left hand side of the crankcase to the oil tank. Bike was leaking oil from the main seal, and since I noticed that this was the case only when the bike had been run on the freeway, but not in stop and go city traffic, I suspected this was a breathing issue. I inserted a PVC value in line on the breather pipe, and this reduced the oil leak but did not stop it. I then read on the Norton Club site about better results with a brake booster valve (motormite/Help power brake booster valve vs. a PVC valve) and installed one of these. This reduced the leak somewhat again, but I am somewhat concerned about the valve itself. It is made out of an ABS type plastic, and has a rubber flap which covers a circle of holes to stop back pressure. It reacts somewhat faster than a PVC valve, which may be the important thing since the back pressures seem very light (estimated to be about 2 psi in the article on the Norton Club site) but with the heat of the engine oil I am somewhat concerned about the longevity of the valve....