Don't worry about the cover layer. Automotive leather is extremely durable, using a soft brush similar to this one...
http://www.pingens.se/p/interior-22/interiorborste-pro.html
...will not damage your steering wheel. Same for leather seats: water, soap and brush. Nice matte finish returns. If very dirty, special melamine sponge can be used. Here you need to be careful, the melamine sponge might damage the top coat of the leather if you press to hard.
http://www.pingens.se/.../...e-e38-interiort-och-slutfinish-del-3.html
/M
