The check engine light turned off itself about 50 miles after it came on, same tank of fuel. It must have been an anomaly.
Just to be sure, when the tank was empty, I put in 5 gallons of 87 and ran that to empty.
My observation on running on straight 87 octane gasoline (10% ethanol) is that the engine runs about the same, but I did notice some knocking under hard acceleration. It is not something you notice on ~90 octane ~E25. The car accelerates very smoothly and quietly on E25.
I have the scanguage II, I'm looking at ignition timing, and I did not notice any change in timing from E25 to E10 and back. The ECU should be changing timing in order to deal with the lower octane rating of E10, but it is not noticeable at the frequency that the scangauge monitors ignition timing.