This website has very good pictures of various spark plug conditions.
http://www.autolite.com/media/11838/plugtips.pdf
Post which one of the examples most closely match your sparkplugs. Then we can zero in on the root cause.
I hope that the "eating" is not detonation.
Also, 91 octane (US) fuel is required.
Note that octane measurements are different between the US and Europe.
- Research Octane Number = RON
- Motor Octane Number = MON
The European octane rating is RON, whereas the US octane rating is an average of (RON+MON)/2.
So, US octane of 91 ((RON+MON)/2) equals EU octane of 95 (Research Octane Number = RON).
Kippis!