Ethanol has a higher oxygen content in it than your normal fuel, which will lean the car out. I have seen examples of cars tuned for 98RON at 11.8:1 AFR's and when filled up with E10 they leaned out pretty badly (this is also evidenced in the HPI GTR special that came out a few weeks ago). Octane rating has nothing to do with it, its simply the fact the fuel has alot more oxygen in it than normal, and for the car to run properly on it, the ECU needs a retune. Wether the stock ECU can compensate for such a rise or not, I have no idea, but I dare say that may be why Nissan reccomend against using it.