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Dude, it's on the other side of the wheel where you can't see it. Screw it!!!
Seriously though, some warm water with dishwashing detergent in it to remove all the ground in oil and dirt. Then follow it up with some Adam's Leather Cleaner. Then follow that up with some Adam's Leather Treatment. You'll be good for another few years when you notice your hands get dirtier when you drive than when you work.....