OK, here is a site with a great explanation on tires and how they react to different products. I saw this same explanation on a show but couldn't recall all the facts. Here's a snippet:
"Tires/Rubber Trim: There are two main degrading agents that attack tires. They are UV light waves and ozone. Both of these attack the long hydrocarbon chains of the rubber and by breaking these chemical bonds, shorten the molecules with resulting loss of elasticity and other problems. Tire manufacturers add two primary sacrificial protectants to the rubber. To protect against UV, they add carbon black. This is why tires don't come in designer colors to match your paint. The carbon black will turn white/gray as it absorbs the UV and dissipates the energy as heat. Thus the basis of rubber parts turning gray as they age. To protect against ozone, tire manufacturers add a wax based sacrificial protectant. The ozone attacks the wax and depletes it. As the tire rolls, additional wax is forced to the surface of the tire. This is referred to as "blooming". This blooming refreshes the surface wax protectant. A tire that has not been flexed will have the wax depleted by the ozone and thus begin to degrade and suffer "dry rot". The silicone oil in Armour All et Al may actually dissolve the wax and be the cause of premature tire side wall cracking/failure. It is rumored that some tire manufacturers will not honor warranties on failures caused by silicone based products. I am in the process of checking with the major tire manufacturers to determine the validity of this rumor. In conclusion, any tire dressing should contain a UV protectant to bolster the efforts of the carbon black and preferably not contain any silicone."
More info:
http://www.unofficialbmw.com/all/carcare/all_dont_use_silicone.html