I'd keep well away from any petroleum product like wd40 or kero....they also attack the sealant around the tyres as well as attacking the tyre itself and if you hit any of the brake disc surfaces it will effect your brakes & pads.
Spray Furniture polish or 'Son of a Gun' works perfectly and won't contaminate any of the brakes...use an old toothbrush on the centre spine and around the spoke holes.
The polish as said slows the build up of new muck