Is a "higher flow" reed valve made of a softer material so that it flexes open more?
Or are the manifold and reed larger?
I can't think of any other reason that you would get carbon build up (So Quickly).... Air+Fuel+Oil->Explosion If any of these is impure than you would get some byproducts...unless the air or fuel is dirty (unlikely), then I would guess that its the oil...right?
I'm no expert on this, but that's just my general intuition.
And yeah the synthetic is more... but a quart lasts me about a year (Maybe 1000mi) in my scooter.