Well, if you do have a belt that squeals, applying some belt dressing will remove the noise instantaneously. Of course, some people may frown on this practice because it is more cosmetic for the purpose than anything. If the belt squeals, then it is too loose or too tight. Ergo, the correct solution would be to tighten or loosen the belt.

On the other hand, I was thinking that application of the belt dressing as part of routine maintenance could prolong the life of the belt or at least make it more supple. But I really don't know if it will. It was just a thought. After all, we do apply dressing on the rubber hoses of our engines as part of maintenance/detailing, don't we? So, why not do the same for the belts?

BlueBimmer, you spray the dressing on the surface of the belt that has contact with the pulley. Actually, the instructions are on the can's label.