Reasons not to use gas containing alcohol (ethanol)

1) alcohol has less energy to burn per same volume as gasoline. So the jets have to be larger when using ethanol (gas with alcohol) but still an engine using pure gasoline is more powerful. (3% larger jets with E10)
2) alcohol dries out rubber (crank seals, float valve tip on good carburetors, fuel line). A dried out crank seal will leak and need replacement. I unknowingly used 25% ethanol gas (because it's unmarked) in my Suzuki 100 (brand new) and had to replace the seals after only 2 years.
3) alcohol removes some of the oil from the cylinder which lessens ring protection (so you have to use more oil for normal protection)

More than 98% of U.S. gasoline contains ethanol, typically E10 (10% ethanol, 90% gasoline), to oxygenate the fuel and reduce air pollution. E10 gas is available in all octane grades, including regular-87 and premium-91. Low-octane gasoline is blended with 10% ethanol to attain the standard 87 octane. Ethanol contains less energy per gallon than gasoline, to varying degrees, depending on the volume percentage of ethanol in the blend. Denatured ethanol (98% ethanol) contains about 30% less energy than gasoline per gallon. The higher the ethanol content, the higher the octane. 100% ethanol has an octane rating of 113. Ethanol fuels expire in 90 days. When exposed to excessive water they may expire even sooner. Studies have shown that after only 100 days, alcohol fuels absorb enough moisture to phase separate (separation of alcohol and gasoline).

The gas pumps should show the E rating (E10, E15, E85). If no alcohol-free gasoline is available then you can do this to take out the alcohol:
1. put the gasohol in a gas container
2. add 25% water and shake the container for mixture of water with alcohol (they attach to each other)
3. let the container rest for 30 seconds for the water/alcohol to settle to the bottom
4. slowly invert the container and loosen the cap so the water/alcohol comes out (hopefully into another container which you can empty onto concrete or asphalt outside for the alcohol to evaporate). Water/alcohol will be mostly clear but the gasoline will be colored so you can see the separation line.
5. turn the container upright, let it sit 10 seconds, pour its contents into the fuel tank or another container but leave the last couple of tablespoons of fuel in it because it will have a little bit of water/alcohol.

Go here to find a gas station that sells ethanol-free gas:

compression ratio - octane needed - cranking psi per compression ratio (with good rings)
CR --oct - psi
5:1 - 72 - 79
6:1 - 81 - 109
7:1 - 87 - 137
8:1 - 92 - 164
9:1 - 96 - 189
10:1 - 100 - 213
11:1 - 104 - 236
12:1 - 108 - 257

If you haven't done the measurement-calculation to know your engines compression ratio then you can go by the cranking pressure but that has its drawbacks since pressure depends on ring end gap and how well your head gasket is sealing. If you are looking to buy a compression tester you need to know that most of them cause a lower psi reading for small engines because the part that threads into the spark plug hole is usually shorter than the part of the spark plug that screws in there. This leaves additional space which lowers the compression ratio. Looking on Amazon I found this gauge that isn't expensive and whose threads are longer than normal (9mm):