Where in the world could so many weird ideas come about about a couple of tires that are not going to be used for excessive speeds. Yes different rubber formulas make a difference on how long a tire will last in or out of the sun. For most plastics and rubber formulas the biggest killer is sunlight. I have several trailers that came with 8'', 10'' or 12'' tires and wheels. I have picked up an assortment of 12'' tires from the wrecking yards to have for spares or replacements. I often throw away any thing smaller than 12''. Harbor Fright has tires and tubes which can be used for wheelbarrows and smaller tire setups. If I have any questions about a particular setup I often will add a tube which can last for years provided heat build up from highway use is not a big factor. My larger trucks have split rims and tubes that have lasted for twenty years with out any issues. I have often been surprised at what tires fail and what ones I thought should have failed, but have not. Thanks