I thought I would mention, the type of finish on these machine is not even slightly related to the traditional Japan finish which is actually an Asphalt based product ( IE petroleum distillate.. think tar ) The krinkle finish is a PAINT.. and I know that leaving oil on it WILL stain it and weaken the chemical make up. I still will not use OIL as a cleaner and worse yet I do not cover the entire surface with it. YES it does make it very black.... but you would not do that to your car dash would you ? many things (especially black things) get dusty looking and tend towards grey, putting a liquid on them makes them black, but I don't think it is the right thing to do .
Also of some note is that there are very very few decals on this type of machine, and because of the wrinkled surface they are not adhered all that tight. Don't rub or brush them, best to avoid touching them at all. I am tempted to try "Armorlall" but I don't like the silicone based nature of it, over time that builds up and has a drying effect on vinyl, have not researched it on paint.