Avoid silicone based products, they are rather harmful, although it might seem different at first sight.
The long hydrocarbon chains that make up the rubber of the tires are attacked by both ultraviolet rays and ozone. As the molecules break up, the rubber loses its elasticity. The manufacturers use carbon black to protect the rubber against UV rays (this is why we only have black tires). A wax based protector is also used: as the tire rolls on the road, additional wax is forced to the surface and absorbes the harmful effects of ozone.