I just got off of the phone with the OS Engine Techs at Hobbico, they said there are two reasons for the line from the crankcase to the muffler, for a cleaner installation, all the oil comes out in one place, and to stabilize tank pressure taking advantage of the positive crankcase pressure on the intake stroke and and the even greater crankcase pressure during the combustion stroke. The ABC setup is not immune from blowby, therefore lower end lube is accomplished just like a ringed engine. The cylinder is a mildly tapered bore not as extreme as a two stroke tapered bore yet still has a noticeable pinch at the top. In the instructions under Engine Construction it says "the cylinder bore is a little tapered" I could not copy and paste it or I would have.