I have a car (MG3) which is compatible with E85, E20, 91 and 95. I did some research and this is what I came up with.
95 is 100% petroleum based. 91 is 10% ethanol, E20 20%, and E85 85% ethanol.
I was told by several neighbors that E85 gives worse KM's per liter than 91 or 95, which negates any savings due to the dramatically lower price. I didn't believe it so carefully recorded Km readings and liter usage with several tanks of 91, E20 and E85, driving in a similar manner as much as possible. With my experience the range of a tank of E20 vs 91 was indescernible. A tank of E85 netted a few less Km's, but the extra range from 91 was absolutely not enough to negate the savings vs use of E85. E85 was by far the most economical choice. I my car runs fine on all of the fuels. I definitely can not notice any difference in performance with a tank full of E85 vs 95.
One problem with E85 is limited availability. In my area it's only available at the larger PTT outlets. I've never seen it available at a Shell, Esso or Caltex station. But if you're not able to find it the obvious answer is to use another fuel type.
Another problem and the main reason some cars are not rated for E20 or E85 is that ethanol is more corrosive than petroleum. So if ethanol corrosion rated materials are not present in the fuel system E20 & E85 definitely should not be used. Myself, if I leave the car sitting for long periods of time (when I leave Thailand for multiple months) I make sure it's full of 95, and ask the neighbor who takes it on occasional runs to keep it full of 95.
As far as environmental friendliness, from what I understand ethanol burns more cleanly than petroleum. But burning ethanol also produces more greenhouse gasses, so it's probably a wash.
The ethanol used in Thai motor fuels is predominantly distilled domestically from cassava or sugarcane, so to me I feel better about supporting the Thai farming community vs the petroleum industry.
The reason that the ethanol fuels are cheaper in Thailand is because the ethanol industry is Thai government subsidized via import taxes on petroleum. So E85 and E20 fuel's cost advance could disappear overnight with the stroke of a pen. But for now, it's the best option IMO for cars that are driven daily.