This thread got me curious, I have used both 89 and 91 octane in my Eos and could not tell the difference during normal driving --- from the data on Octane I have copied in below, it seems that you get lower maximum horsepower with the lower octane gas, as a result you would have to spin the engine at higher rpm to get the same acceleration time. If you don't drive your Eos hard then you probally will never see any difference with the lower octane gas.
Bottom line - if VW supported use of 89 octane fuel then they would have to claim a lower horsepower output for the Eos and would have to claim slower acceleration times.
Here is the geek speak regarding Octane, copied from the Wikipedia Octane Article:
Using a fuel with a higher octane lets an engine run at a higher compression ratio without having problems with knock. Actual compression in the combustion chamber is determined by the compression ratio as well as the amount of air restriction in the intake manifold (manifold vacuum) as well as the barometric pressure, which is a function of elevation and weather conditions.
Using high octane fuel for an engine makes a difference when the engine is producing its maximum power or when under a high load such as climbing a large hill or carrying excessive weight. This will occur when the intake manifold has no air restriction and is running at minimum vacuum. Depending on the engine design, this particular circumstance can be anywhere along the RPM range, but is usually easy to pinpoint if you can examine a printout of the power output (torque values) of an engine.
When this occurs, if a fuel with below recommended octane is used, the engine will knock. Modern engines have anti-knock provisions built into the control systems and this is usually achieved by dynamically de-tuning the engine while under load by increasing the fuel-air mixture and retarding the spark. For a specific test case, the engine maximum power was reduced by about 4% with a fuel switch from 93 to 91 octane (11 hp, from 291 to 280 hp). If the engine is being run below maximum load, the difference in octane will have even less effect. For each 1000 feet of altitude the atmospheric pressure will drop by a little less than 11 kPa/km (1 inHg). An engine that might require 93 octane at sea level may perform at maximum on a fuel rated at 91 octane if the elevation is over, say, 1000 feet.
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, Canada 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, and even deliver 98 (RON) or 100 (RON).
