Well, the Owners Manual for our 2014 Prius
emphatically states NOT to use anything
higher than E15:
• Use only gasoline containing up to 15% ethanol.
• DO NOT use any flex-fuel or gasoline that could contain more than 15% ethanol, including from any pump labeled E30, E50, E85 (which are only some examples of fuel containing more than 15% ethanol).