I'm not saying one has anything to do with the other but here are the facts - I have always used premium gas in my '08 G35x sedan. But the difference in my area between regular and premium rose to a $10 difference per fill up and I cheaped out putting in 3 fill ups of regular grade. On my second tank the brake and battery light came on for a few minutes - I went to a battery shop where the guy correctly told me those two lights usually mean a bad alternator, but he tested the battery and it was OK. During the second and third tank of regular those lights started coming on more frequently, finally coming on every day. I made an appointment with my mechanic. Three days before my appointment I filled up with premium, not b/c I suspected any relation between regular gas and the dashboard lights, but b/c the car did not seem to be running as well, The lights never returned after the premium fill up. The mechanic load tested the alternator and it was fine. I've had the car for a week since the fill up and the lights never returned after coming on every day for the last week with the third tank. The mechanic could not relate fuel grade to the lights but told me not to use lower octane gas. If it's a coincidence it's a weird one.

 

I appreciate the trolling (really I do) but you're trying to run before you can walk here. It's actually perfectly fine to use 87 in any modern car, and OPs issues are purely coincidental. The only thing he's doing by using 87 is asking the ECU to retard timing, thereby robbing him a bit of HP and possibly a little fuel economy.

 

you're right it was late and I was half asleep with several beers in me. Carry on

 

Since the car is tuned to run on 91 you actually lose out of fuel efficiency and may run into an issue is pre combustion. But will still they less gas efficiency off set the 10$ more a tank hard to say. Also what does it do to the longevity of the car.... All important questions