Of course, there is no official documentation for this.
So, I have surmised this information by reading all of the posts regarding: "CVT Fluid" inside of this forum; also by observing what it happening to my own CVT transmission, compared to other 'regular' automatic transmissions that I have owned in the past.
I could put the links here, but it is probably just easier to do an advanced search on "CVT Fluid", and the "CVT" in general, within this forum / website to get you started reading.
Some members here have noted a higher than normal soot level in their CVT transmission fluid on normal working CVTs. I definitely notice it in my CVT fluid.
In fact, it would be interesting to see the color and mileage of everyone's Mirage CVT fluid.
Below, is an interesting and highly informative Youtube video link regarding automatic transmission fluid changes and soot, entitled: "Here's Why Changing Your Transmission Fluid Can Cause Damage":