mike68:
Trying to clean it is a waste of time, what has happened is the shiny grained outer layer has worn away leaving the dull matt material that makes up the bulk of the steering wheel.The only answer is a new steering wheel or a cover.
This. Or, rather, eventually this. I took a truck over once and the wheel was filthy. All the standard stuff kept bringing off black stuff, so I upped my game and got out the more abrasive options as suggested above. Then I discovered anti-bacterial scrubbing wipes and thought all my Xmases had come at once - so much so I shared the news with a good friend who also used them to attack his wheel. It was only several weeks later that the penny dropped and I realised the black stuff wasn’t muck, and hadn’t been for quite a while… 
…Nice furry finish on the wheel though! 
