RE: Cleaning XPS12 carbon fibre
I also had the same problem. Here's what worked for me:
Take the softest washcloth you have in the house and fully wet it with warm water and get it a little soapy with hand soap (I used Dial gold)
Wring it out until it's damp but doesn't drip. Make sure that the cloth is just damp enough that it will wet the surface but isn't so wet that any water will run into the crack at the edge of the screen.
Gently rub the entire surface of the top with it, paying special attention to the oily bits, rubbing in circles gently until the oily areas look the same in bright light at an angle as the rest of the top.
Now rinse the soap off the cloth in warm water. Wring it out thoroughly and go over the area you soaped earlier and remove any remaining soapy residue.
This method also works well for the touchpad and screen.