The series makes that claim and they have some cred to back it up. They now allow gasoline, gas/electric hybrids, diesel, E10, E85 and now isobutanol.
Today's Examiner article focuses a little more on this story and yesterday's 12 Hours of Sebring, which has been held every year in March for 57 consecutive years. My God, that's longer than Ferrari has been in Formula One racing!
Anyhoo, you can read the full article at this link:
Sebring