Yes, +1 to Dirtman's posts
As said Octane is a HEAT rating, well to be more accurate "Oct" is 8 and "tane" is a carbon chain, so 87 'octane' is a percentage of 8 chain carbon molecules, very simplified.
8 chain are more stable, less likely to self ignite when heated, so the more "octane" molecules the more stable
A gallon of 87 octane gasoline has EXACTLY same energy as a gallon of 93 octane, but read on
Ethanol can bond to the 6 and 7 chain carbons to make them more stable, so ethanol increases Octane rating
But ethanol has less energy per gallon than gasoline
So 10% added ethanol would lower MPG a bit, not 10%, maybe 2%
15% even lower MPG
Ethanol can also bond with WATER, gasoline can not, all gasoline will have some water in it, in the winter this water can freeze in the fuel lines or filter and cause issues
So winter gasoline will have ethanol added which prevents the water from freezing