Saint Crispins also suggested yearly cleaning with saddle soaps. My shoes so far are still alive and well, but then they are not 10 years old yet...
Also Patrick et al, what do you think of using Saphir Cleaning Soap or Saddle soap before doing my first Reno?
From the Hanger Project website it does say the soap is great at removing polish build up and excessive pigments, so I'm thinking it might lighten the shoes simply by removing the wax polish that has built up over time.
Saddle Soap doesn't remove polish well IME. But it all depends on how thick the polish is.