I just got mine two weeks ago and it is printed right on the inside of the fule door, 91 Octane Recommended, Minimum 87.Having said all this I was completely UNAWARE that our little 500s had a 91 octane requirement. The dealer told me 87 and like a fool, I took him at his word. Guess I'll go check the manual today. Hopefully, this is the case, because it means there's a little more poke available in this engine that I have yet to exploit.
I have been running 87 since day one and haven't noticed any problems. That being said, I filled up with 91 from empty and took a 350Km trip yesterday. I can't say I noticed a big difference in out right power but the car deffinitly seemed less laboured, especially when climbing hills without a down shift. I also recorded 5.1L/100 or 46.2 mpg avg on the trip without trying for mpg. Going to keep an eye on mpg. If the increase pays for most of the fuel upgrade ($4-5 per tank) then its a no brainer. I'll shut up now