So here is the question. Where I am at I can get premium, or premium with " up to 10% ethanol". Now clearly I would love the straight premium, however it is .35-.40cents more expensive. Mainly because pdq is always the cheapest gas around the city. Will running the ethanol gas be harmful for my 04 a8l? Is there a big enough impact in mpg that will negate the difference in price per gallon?

 

Your 04 will run fine on either. It will get about 10 to 15% better mpg on fuel without 10% ethanol.

 

Seems to be a wash on the difference in price compared to MPGs.
I always run pure gas if I can get it.

On a trip recently they may have had that 15% garbage [not properly labelled] at a station because my mileage tanked badly....

As feirstein said it will run fine, but the fuel won't last more than 30 days without separating.
Keep it fresh with small fill-ups if you don't put a lot of miles on in a month.

 

Well i fill up weekly, so i guess ill give the ethanol a shot. And if it last almost as long as the pure stuff then i great. If not then i guess i got some math to do!