I, like Lostazhell , have used dish soap or liquid hand soap with wet paper towels to get that stuff off. Be sure to rinse the wall when you're done to get rid of the soap, and dry it off. (it takes a lot of paper towels)
I'm not saying that this works BUT I'm amazed at all the things that WD-40 takes off. I just showed someone how well it works on what's left after you remove a price sticker. It would be interesting to see how it works with BX coating on a painted surface. I do remember ruining a silver embroidered skydiving windbreaker by carrying a roll of BX. That was an expensive lesson. Joe
Nearly all AC, MC, FMC used today has an outer jacket made fron aluminum. I agree that the oil is there as part of the manufacturering process.
Aluminum does, in fact, react with air. This results in a layer of aluminum oxide (think 'aluminum rust' so to speak)forming on the surface of the aluminum. While this layer doesn't flake of as real rust flakes off of iron, it does come off fairly easily. The acids in your sweat combine with aluminum, which is why your hands get so black when you work with MC.
Now .... think for a moment .... what do you get when you mix fine colored particles witha light oil? We have a special word for this mix. We call it 'paint.'
So, in handling the MC we make what amounts to a black paint- which then rubs off onto the wall. Remove the oil, and the black stuff - aluminum oxide- remains.
Unless the surface is completely smooth and non-porous, you're stuck with having to cover your marks with more paint- paint that matches the wall this time.
I always carry a spray bottle of ZEP glass cleaner mix and a roll of white paper towels. I mix the ZEP myself from concentrate and like to make it a little strong, so it will remove oils like that MC gook and oxide, but not so strong that it takes latex paint off walls. I use it for cleaning just about everything. Every light fixture and wall plate gets cleaned after installation when I do the finish on a new house. Watch out for those paper towels with the color prints on them though. That print color stuff [Soy ink??] comes of on the walls and leaves a nasty stain that is extremely difficult to remove.