leaded fuel isnt bad for your car, it just burn substancially hotter, which in turn can melt your catalytic converter as well as create lead deposits in your O2 sensors. If you put 105 in your car only once, it won't do any harm.

To get back to your questions about regular vs. premium, there is nothing harmful about regular, but your car runs fairly high compression with quite a bit of timing, so you really want to run higher octane. Your car can compensate for lower octane fuel for some degree, but not really all that much, so it's not something i would recomend, especially if your hard on your car. The actuall differences in the gas are some of the additive packages and detergents, but realistically, the only main difference is the octane raiting.

The thing you really don't want to do is put diesel gas in your car, that would suck!!!!

Later,
Aj