Yep, that's THE question. For starters, I WOULDN'T use anything Armour-All. Go on any classic car forums and there are guys with sad stories about how it dry rots tires and plastics way more than any protection. Been there myself. It's probably the worst. I guess if you burn through street tires fast, it doesn't matter though. They'll be worn out faster than any damage. But some guys don't put a lot of miles on their rigs, and in that case, I'd steer clear of 'miracle' spray-on tire shine products unless it's shown they don't cause dry rotting. I use WD-40 on my tires (applied with a rag). Lasts long enough looks natural ...doesn't dry them out (probably does just the opposite). It's not for everyone. Doesn't last as long as tire shine stuff, but cheap to re-apply...