Short Answer:
I wouldn't worry about it. I'd just stick with premium and avoid gasolines with more than single digit quantites of ethanol in it.
Long answer (if you hated chemistry, skip):
Leaded gasoline contains tetraethyl lead. It's an "octane booster" that makes it easier to achieve higher compression without premature ignition (more power!). The octane rating of gasoline (87, 89, 93) is the amount of compression the gasoline can take before "knocking" when compared to isoctane gase (100). You can actually have octane ratings above 100.
Tetraethyl lead was the first widespread octane booster. It was gradually replaced by better, more environmentally-friendly alternatives (MTBE and ethanol). MTBE is falling out of favor due to groundwater contamination and is generally being replaced by ethanol in 1-5% blends. Ethanol can provide for more complete combustions, but it raises the vapor point during warm months (leading to smog) and ethanol is more corrosive to certain types of seals.
In general, just go with the highest octane gas you can find and avoid ethanol blends and you'll be fine. Unleaded gasoline will cause SLIGHTLY more erosion around the valve seats, but it will take tens of thousands of miles to show any effect.
