Buying gas in Spain is much like in the States; the pumps are just about the same, except the money is in Euros and you buy it by the liter.

All grades of unleaded petrol (benzin), diesel (gasoleo 'A') and LPG are available as well as lead substitute additive. Leaded no longer exists. It is allowed to carry petrol in a can. If you make a mistake refueling, don't start the car but ask the attendants to drain the tank for you. This will save you from having to pay for any damage.

Keep your tank topped up in case gas stations close during the siesta hour. However many gas stations are self service.

Credit and debit cards are widely accepted, although they probably won't work at automatic pumps, which are often the only pumps open out-of-hours and at lunch-time (from noon to 3pm) away from the Autoroutes. It's a good idea to let your card issuer know you will be traveling abroad. This ensures they don't suspend your card if they spot it being used in unfamiliar places, which they sometimes do as an anti-fraud measure.

Translations:
Leaded = super or super 98
Unleaded = sin plomo 98 or Eurosuper 95
Diesel = gasoleo