I've been noticing soot in certain areas of my MacBook's( late 2007) key pads. I've been using a silicon key pad skin. It seems heat needs to vent out of those keys. Now I want to discard the silicon protector. But I really do not want to wear out the keys. I've worn out my iBook's letter "A" before. How durable are these key pads? Do they wear out like those in the iBook G4?