http://en.wikipedia.org/wiki/Octane_rating

  In most countries (including all of Europe and Australia) the "headline"
  octane that would be shown on the pump is the RON, but in the United
  States and some other countries the headline number is the average of
  the RON and the MON, sometimes called the Anti-Knock Index (AKI), Road
  Octane Number (RdON), Pump Octane Number (PON), or (R+M)/2. Because of
  the 8 to 10 point difference noted above, this means that the octane in
  the United States will be about 4 to 5 points lower than the same fuel
  elsewhere: 87 octane fuel, the "regular" gasoline in the US and Canada,
  would be 91-92 in Europe. However most European pumps deliver 95 (RON)
  as "regular", equivalent to 90-91 US (R+M)/2.

In high altitude areas of the USA you can purchase 85 AKI fuel which most
normally aspirated cars happily digest with no problems.  My 5000CS gets
87 most of the time and is glad to get it...

Interesting.  I wonder if the article meant to say "silicone" rather
than "silicon".  Silicon is an element resembling a rock.  It won't get
past the fuel filter in any quantities that would damage an O2 sensor.  
Silicone, on the other hand, can be an oil that's soluble in gasoline.  
It is very heat resistant but it will still burn into a powdery mix of
silicone dioxide and silicon carbide.  Even small amounts of silicone in
the fuel could wear down engine parts or glaze over an oxygen sensor.