just a little info.
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, 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.
woot i like that last sentence
As your in Canada and I'm in the UK, the SLK is a foreign car to both of us...
USA and Canada use (RON+MON)/2 to define the anti-knock index or AKI. The difference between RON (Research Octane Number), and MON (Motor Octane Number) can vary from blend to blend and is called sensitivity.
The AKI approach may be thought of as a better descriptor of octane quality, but it's by no means universal. Outside of the USA and Canada, I think the rest of the world uses RON as the headline figure at the pump.
In Europe, EN:228 proposes a minumum of 95 for RON and 85 for MON. So a sensitivity of 10 octane numbers might be expected, but real fuels can be different and still meet the regs. For example, sometimes the refinery has to 'give away' octane in order to comply with other properties defined in the specification.
If you fancy a more detailed read about octane, have a butchers at this:
As for 'knock sensor equipped engines may give more performance'. Strictly speaking, if the engine were perfectly calibrated there would be no benefit on running it on fuel with higher otctane quality than the calibration fuel. The knock sensor only provides a signal when the engine knocks on fuel that has insufficient octane quality to prevent knock.
In practice, some engine calibrations have some headroom and therefore the engine is able to benefit from higher octane fuel. This isn't due to the knock sensor.
Just trying to provide some clarification BTW.