To Herk, ethyl added fuel can contain "up to" 10% of ethyl. Flex fuel marked vehicles are designed to burn E85 or regular gasoline. E85 is 85% ethylol. I realize thats not the topic, but I wanted to clarify. I will be putting my solera to bed soon, so can't really help on this post till spring. I have burned the 5% bio stuff, and didn't notice any lower fuel economy. May I suggest, for those who keep detailed fuel records, note bio, %, and what you observe during that tank use. Best, someone bring this post back early next year, so we can see what, if any, issues occur through the records.