Where I got that particular piece of advice was from a Stihl dealer/repairman - because ethanol and high detergent gasolines were the 2 biggest $$ makers for him as he put it -- they caused the fuel & oil to separate within about 10 minutes of starting.
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The subject of fuel separation in ethanol fuels usually occurs when water get into the fuel. Ethanol absorbs water readily right to the point where it is saturated, then the ethanol/water mixture will separate from the gasoline. Your chainsaw won't run on the ethanol/water mix, and will run badly on the remaining gasoline because the ethanol provides a needed octane boost to the gasoline. A lot of the stories out there about ethanol are simply wrong. Try this: put a cup of mixed ethanol gasoline and 2-stroke oil in a glass jar and seal the cap. Now wait a day. See any change? Now remove the cap and check the fuel after another day. See any change? Ethanol will pull the water vapor right out of the air and looks a bit like skim milk.
Ethanol/gasoline mixes will work very well if you keep the water away from the fuel. Only mix enough fuel to be used in a week or so, and store it in a sealed container.
A far greater risk than ethanol is lending your saw to a neighbor, friend, or, worst of all, to a family member. I can't count the number of borrowed saws I've seen that have been run on straight gas, or a bar oil/gas mixture, or the fuel tank filled with bar oil. The bar oil just causes a temporary problem, the straight gas gets very expensive.