Well when the Fieros first came out there wasn't a lot of corn gas around. But in Florida non ethanol gas was $4.29/gallon (93 octane) and 87 octane is $3.47/gallon. The only thing non ethanol gas will hurt is your wallet.
Any Regular 87 octane is what you should run. As Lar mentioned above there wasn't all that much corn fuel around when Fieros were new - a lot of stations still even had leaded fuel at that time to satisfy people who still used that type fuel. I've seen no noticeable differences in my '86 GT personally. At 285,000 miles it still gets around 23/24 MPG combined city/hwy.
However don't waste money on higher octane blends for stock Fieros. The ECM doesn't act like modern ECMs where you can sometimes gain some small additional HPs or power based on going up on the blend. You get around 130 HP out of your 2.8L on 87 Octane? You're gonna get around 130 HP out of your 2.8L on 93 Octane. Obviously this doesn't apply to individuals that are running drivetrains and ECMs that are tuneable for specific blends.
The octane rating tell you how much energy it takes ignite the gas, not how much energy it will deliver. That value is essentially the same in regular, mid-grade, and premium gas. You can damage your vehicle if you use a fuel with a lower octane rating than the engine needs, but you won't derive any benefit from using gas with a higher octane rating. In other words...use the lowest octane that doesn't give pre-detonation (pinging, knocking)....which for most lower compression engines is 87 octane.
