Living in an area particularly exposed to traffic, I am not surprised at the results of cleaning out my air purifier. After a year, the big problem is not dust as it was in Korea for me.
When I bought the Daikin air purifier less than a year ago, the insides were white, and one of the filters was gray. It was black when I opened it up. Unlike dust, it did not just wipe off. It was oily, like I was cleaning the grease off of a mountain bike chain or the insides of a petrol engine.
I wonder what my lungs are like now… can human lungs get rid of this oily grime any better than an air purifier can?