Can i use normal unleaded fuel for my 1997 WRX?

I bought a 1997 WRX 3 years ago with about 160,000 miles and i have put about 13,000 miles on it since. Since i bought it i have always used normal unleaded fuel but my friends have told me before that using normal unleaded fuel could be really bad for the engine. I think the previous owner used normal unleaded too and i have found on the net that some people think it' fine but some say its not so i want to try yahoo answer and see what you guys think.