Aluminum oxide isn't usually much of a problem, other than aesthetically. The oxidization of aluminum is a self-limiting process, so that as long as the environmental conditions don't change, it will proceed very slowly.
Search the forums for Turtle Wax or Aluminum Polish, and you'll find several threads on how to make it look nice and shiny again. However, it is a lot of work to put into something that's not going to make a functional difference, so I wouldn't bother unless this is a collectible-quality bike
