Here's what Petro Canada website has to say on the higher octane issue:

What octane does my vehicle need?
To determine your vehicle's octane requirement, look at the manufacturer's recommendation in your owner's manual. Most auto manufacturers recommend 87 octane gasoline, as measured by the (R + M) / 2 method on a test engine under defined operating conditions. If the vehicle knocks on the recommended grade, a higher octane grade should be selected. Some foreign vehicle manuals recommend a Research Octane Number (RON) instead of the more common octane rating that appears on most gasoline pumps. As a rule of thumb, the recommended octane rating can be determined by subtracting four (4) from the recommended RON number. A vehicle that calls for "91 RON" should use 87 octane gasoline (as measured by the (R + M) / 2 method). Using a higher grade than is required will not usually increase performance. However, if the vehicle is equipped with knock sensors, as many late model vehicles are, a higher octane grade may enhance performance.

Why does my vehicle require a higher octane gasoline than is recommended in my owner's manual?
There are two reasons. First, the engine may be at the upper end of the octane requirement range for the particular model. It may, therefore, knock during periods of heavy engine load. The octane requirement for each engine in vehicles of the same make is different because of variations in manufacturing tolerances. Industry testing has shown that this difference can range between two and five octane numbers. While you will not hear knock, the spark timing will be retarded, resulting in loss of power and performance.

The second reason for using a higher-octane gasoline than recommended is because the equilibrium level of combustion chamber deposits is higher than average resulting in a higher than average octane requirement for the engine. Industry testing shows that the octane requirement increase (over the initial 20-50,000 kilometres) can be as much as nine octane numbers. This means that a vehicle that was originally designed for 82 octane fuel may eventually require 91 octane fuel to perform satisfactorily.

Petro-Canada's premium gasoline, SuperClean*, contains a higher level of Tactrol*, our exclusive deposit control additive, than our other grades and will help clean up deposits left by lesser gasoline brands.