Newer than 1996 can handle E20 with no issues. If it is a Flex Fuel car, then the sky is the limit. After that it "depends". The problem with older vehicles is the fuel system components were not designed to tolerate Ethanol. Also, who is doing the blending? What is the purity of the Ethanol that is being used to blend? I blend my own stuff and have no issues with 20-25% Ethanol so far, other than a reduction in gas mileage, but you do get an improvement in performance, especially if your vehicle utilizes a knock sensor.
Stock Fox I would be leery of using more than E15-20. If you address the fuel system components and can tune for it, then no worries.