Due to the dust and oily nature that I had noticed in the Kaytee aspen awhile ago, I switched over to hemp bedding. ---Virtually no dust (a little at the bottom of bags, no oils, and it's a renewable source (more so than aspen). I noticed increased appetites in my snakes (my corns have always had solid appetites, so mainly an increase with my ball python), lower odor with feces, while they can still burrow and more complete sheds (when compared to before). <--I switched to sani chips for awhile and it seemed like they were drying out the snakes more.
Anyways, I know this thread is about aspen, so here are my suggestions/opinions for oily/dusty aspen:
1. The oily occurrence in Kaytee may be because the aspen is leaching the oils because it is in a plastic bag. Kaytee does put those tiny holes throughout the bag, in order to prevent the oils from leaching out with extra air flow, but depending if the bags are stored tightly in sealed boxes with little to no air flow for the tiny holes in the bags to be effective and possibly in hotter weather, the oils are leaching out from the wood. What you may want to due is transfer the aspen, after you buy it, to a brown paper bag--like a grocery bag. This would allow the aspen to 'breathe' and the paper would help to absorb the oils. Have you noticed that the non-retail brands of aspen are packaged and shipped in brown bags, ^^that could be the reason why. Additionally, batch to batch of aspen may very too.<--I am only hypothesizing on the oils, but I had noticed it.
2. As far as the dust goes, here's what I did: I screen cleaned the aspen before I put it with my snakes/mice/rats. Basically, you can get a metal screen (the bendable, firm type--if its flimsy screen, you would have to build a frame for it) with fine holes in it (kind of what is used for window screens--found in Home Depot/Lowe's), cut a rectangle out of it, and bend up the corners. Then place the aspen on the screen and shake it outside or over the trash, like you were panning gold from a river (y'all on the west coast know what I'm taking about; ^^yes I said y'all, that's how we talk in North Carolina). As long as the screen is fine enough, any dust will fall through, but the aspen shavings will not. Or, if that's too hard, you can use a scoop, like one you would use to scoop feces out of a snake's viv, as long as it is the metal screen type (like what's sold at Petsmart in the reptile section). I use more of a cat litter scoop for removing feces, which is plastic and has long slits in it, which does not work well to filter the dust from the aspen.
3. Yes, the screen filter takes a little time, so a more simple way is to just open the bag of aspen across the top, then tap the sides of the bag, and then shake the bag. If you do this a few times each time you get aspen out of the bag, the dust will slowly filter towards the bottom. At the end of the bag, you will be left with about 1/8" of dust (depending on the size of the bag), which you can then just throw away.
Hope this helps.