The "cheap" protectants like ArmorAll, Son of a Gun, and dozens of "quickie tire shine" products typically do more harm than good. The low-grade silicone alcohols in them actually strip out the waxes/protectants that are manufactured into the tires to help prevent UV/ozone damage in the first place.
They work to some extent, but they start a vicious cycle where once you start applying them, you have to keep them applied, or you end up worse than where you started. If you stop, the rubber dries out and cracks. I've had this happen to me personally.
The better products (e.g., 3M, Lexol, etc.) you'll find plenty of information on car detailing sites, etc, way beyond what I could go into here.
However one of the biggest problems with any of these products, is they're a magnet for dirt/dust/pollen/grass. I especially wouldn't use them on a tractor, unless the tractor was exclusively a showpiece, or you want your tires to look like rolling Zagnut bars.
As others have said, limiting UV exposure is the only practical thing that works.
