My uneducated guess is that oil vaporization is negligble. These single-barrels don't pull a lot of CFM and the air path covers a lot of area until the last few inches where it is funneled into the carb. It also travels thru some mesh baffles on it's upward journey after passing over the oil. I would think that if oil vaporization was a problem, you would have to add oil to these things all the time and your neighbors would be after you with pitchforks and ropes for the blue fog coming out of your truck.
