There are two main degrading agents that attack tires and rubber trim. They are UV light waves and ozone. Both of these attack the long hydrocarbon chains of the rubber and by breaking these 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 raw silicone oil that is the main ingredient in most of the nationally advertised over the counter products may actually dissolve the wax and be the cause of premature tire sidewall cracking/failure. The quality tire/rubber dressings should contain a strong UV protectant to bolster the efforts of the carbon black and not contain any raw silicone oil. Many of the nationally advertised rubber and vinyl products also contain formaldehyde. If you plan on having a funeral for your dash, then you may wish to use one of these products.
My personal favorite tire and exterior rubber/vinyl trim protectant is Black Again. It is, in my opinion, the best non-silicone oil based product to restore the jet-black patina to tires and exterior rubber/vinyl. A white creamy polymer emulsion, with a heavy duty UV protectant, it works equally well on all colors of exterior trim. (They tried calling it Any Color Again, but the name didn't work). If raw silicone oil based products have been previously applied to the rubber/vinyl, the raw silicone oil had saturated the material. Black Again will help dissolve out the silicone oil (this is good), but does not seem to be able to do so evenly (this is bad). The finish may be slightly "splotchy" the first couple of applications of Black Again. The only solution is to apply several coats about a week apart, and the finish will eventually even out. An excellent spray on type of protectant for tires and rubber trim is One Grand Exterior Rubber Treatment. Spray on, allow to penetrate and buff off the excess, leaving a medium gloss, non-greasy finish. Harly Tire Nu is another spay on product with a high gloss finish. Meguiar #40 is a vinyl and rubber cleaner and conditioner. I feel it works better on vinyl than it does on rubber. Leaves a medium gloss finish. Sonax Trim Protectant cleans and protects exterior vinyl and rubber, producing a high gloss finish. Zymol Tyre is a new product that leaves a medium gloss and helps restore some of the black patina to rubber. Wurth Rubber Care, Sonax Rubber Care and Zymol Seal are designed for the "live" rubber door gaskets. These products are rich in glycerin and help maintain the flexibility and sealing ability of the gaskets. A nice side benefit is that they help keep doors from freezing in winter.