Higher octane fuel should not be confused with higher quality fuel. So many people put "premium" in their cars because it's labeled "premium" and it makes them feel all warm and fuzzy inside that they're putting "only the best" in their vehicle. It's not a cleaner or more refined fuel, it just has a higher level of octane. Some brands may put additional additives in their "premium" fuel, but that would be the only exception. The short of it is that octane makes the fuel less volatile/explosive. So, if you have an engine that doesn't need reduced volatility fuel, chances are that you could actually reduce that engine's efficiency (power) by using high-octane gasoline. Some engines have higher compression and/or more advanced timing, and running lower octane grades ("more explosive") gasoline can cause preignition (detonation/pinging/spark knocking).
As for the 5.3, it SHOULD require regular 87. I would hear some ticking that sounded like a slight and random exhaust leak when accelerating at full throttle and reaching the upper RPMs. Just for kicks, I tried 93 octane at this last fill-up and I'm pretty sure it didn't do it the short time I drove it. I only use Shell or Chevron and non-ethanol when/where available. I didn't get a chance to drive it much on that tank and my gf has filled it again, but with 87. I'll see if the ticking has returned and try mid-grade if it needs another fill-up.
