2016 Mustang GT with 11,000miles. Get ready for the facepalm. I am not proud of this, but looking to hopefully get an answer. I was running my 93 tune and a friend had convinced me that running 5gallons of e15 along with 11gallons of 93 would be good enough to feel a slight power increase. I was skeptical at first, but I ran it anyways. I ran the tank down to nearly a quarter tank and I received my first code the next morning when the motor was cold. Continued to run the tank down to nearly empty and refilled with 93 completely after seeing the misfire code. Clear code and car runs normal, no loss of power for the entire day. Park it over night, begin my journey to work and on a decel the code comes back. I am at about a half tank now from refilling completely with 93. Clear the code and the car runs normal. Runs normal when the code is present as well. Pulled #8 spark plug this morning and did not notice any oil on the threads. Pulled the next plug up which I believe is #7 and found that it looks exactly the same as 8 did. So I swapped coils hoping the misfire would follow the coil. I had done this several hours ago and will more than likely have to wait until the morning to see if the car will misfire again, because like I said once the car has gotten its morning out of the way, the problem is absolutely nonexistent. Is this a fuel related issue and its working its way out, or is the cylinder getting ready to throw the code every chance it can, causing me to get ready to whip out the check book. Again, not proud of this so please keep the comments on the knowledgeable side.