I accidently put 89 octane in my S500 last week. The mechanic at the dealer said not to worry about it because of the anti-knock sensor. Just don't do it to often.
Well, I can tell you the car has never run worse! Sputtering, uneven idle, etc.
I will never make that mistake again. Now my '84 380SL uses 87 and it likes it, but it was built when 87 was the only option. My feeling is if it says premium unleaded only then that's what you should use!
Sorry about the similes, they were just too funny to resist!