Do you "treat" your speakers rubber surrounds?

Usually only foam surrounds dry rot.

KY is best for rubber

Armour-All deadened the sound of a low value test speaker once. It had foam rubber surrounds, but still the effect was large and not good. I would stay away from anything with silicon.

I would say don't put anything on foam. Rubber however can dry out and crack due to environment and age. I see nothing wrong with treating rubber surrounds with something that will protect, lubricate and repel moisture and dirt. Sounds like this 303 may be worth a shot. After all our skin likes to be well lubricated to maintain its elasticity.

+1 to mink oil or the 303 Aerospace Protectant.