I was an engine man on tornado's and this is indeed bleed air from the engine compressors, which has been used to pressurize the Labyrinth seals of the engine bearings. the air pressure is greater than the oil pressure, so the oil is retained in the engine oil return system, the oil/air mix is then sent though a separator, but not all the oil is capable of being removed though and this smoke is actually a fine oil mist contained in the vented air.
http://en.wikipedia.org/wiki/Labyrinth_seal