Automotive fuel systems, including materials, have been engineered for up to 10 percent ethanol going back to about the early eighties when gasoline laced with 10 percent ethanol (E10) first became available in some parts of the USA.

E10 has about 3 percent less energy than typical straight gasoline blends, which is about 1 MPG on a 30 MPG car or one-half MPG on a 15 MPG car, which means it's tough to measure given all the other variables that effect fuel consumption

My 190E2.6 is stored typically 6 months out of the year for the last 15 years, but there have been a couple of 12-18 mpnth periods in that time frame. Same with my '91 MR2. I fill each with fresh fuel (E10) before storage, don't use any kind of "stabilizer" and have never had any fuel system problems with either car.

Duke