Ethanol is not friendly with older fuel systems as it will deteriorate rubber diaphragms, hoses, Accel. pumps, has a lower boiling point ( think vapor lock), attracts moisture and several other issues including being less efficient (lower MPG).
back in 1987 or so I was visiting Ne. from Calif. and mistakenly filled up with a 5 or 10% ethanol fuel. The car immediately began to run like cr-p and the mileage dropped a reasonable amount. I ran the tank empty as quickly as i could, refilled with normal unleaded and all was back to normal.