I've noticed, here on RCU, that Saitos generally have a lot of blowby coming out the crank vent. Maybe you have an engine with a VERY good ring seating. The amount of oil in your fuel will also have an effect on the amount of blowby. Four-Stroke fuels often have less oil in them (~16%) which won't produce as much smoke or vent expulsion. The advantage I would imagine would be less carbon build-up inside the head and on the muffler, plus a higher percentage of burnable fuel in the mixture.
Personally, I'd try running it a little richer to see if it makes a difference in the blowby. I'd use afterrun oil too. Try placing a plunger on the vent working it in and out to see if there may be some blockage. A piece of crud or machining burr may be hindering the effectiveness of the vent.
Just some thoughts.

Jeff