There is no rule that you have to use high octane petrol in C500 , though its FI but if there is high compression ratio of 1:12 or more then you need such petrol. From what I know, C500 has only 1:8 compression ratio so normal fuel is good. Btw, who told you that you have to use only high octane fuel only in C500 ? , the service guy, have you seen it written somewhere in manual ?