when you shut down the engine, the inner passages of the turbo are still nice and hot, even if you let it cool a little some of the oil inside the turbo will vaporize leaving a small bit of gum or char that will get pushed out next time you start and go into the filter. thats just a LITTLE bit tho, not really measurable except over long periods of time. I wouldnt expect that to lower your oil enough to see on a dipstick thru just a month.