What Kind of Fuel Should I Use?

First, a little primer about octane
The octane rating is a measure of the resistance of gasoline and other fuels to detonation (engine knocking) in spark-ignition internal combustion engines. High-performance engines typically have higher compression ratios and are therefore more prone to detonation, so they require higher octane fuel. A lower-performance engine will not generally perform better with high-octane fuel, since the compression ratio is fixed by the engine design.

The octane number of a fuel is measured in a test engine, and is defined by comparison with the mixture of iso-octane and normal heptane which would have the same anti-knocking capacity as the fuel under test: the percentage, by volume, of iso-octane in that mixture is the octane number of the fuel. For example, gasoline with the same knocking characteristics as a mixture of 90% iso-octane and 10% heptane would have an octane rating of 90.[1] Because some fuels are more knock-resistant than iso-octane, the definition has been extended to allow for octane numbers higher than 100.

As with most things, there are different ways of measuring octane

Research Octane Number (RON)
The most common type of octane rating worldwide is the Research Octane Number (RON). RON is determined by running the fuel in a test engine with a variable compression ratio under controlled conditions, and comparing the results with those for mixtures of iso-octane and n-heptane.

Motor Octane Number (MON)
There is another type of octane rating, called Motor Octane Number (MON), or the aviation lean octane rating, which is a better measure of how the fuel behaves when under load as it is done at 900rpm instead of the 600 rpm of the RON[2][3]. MON testing uses a similar test engine to that used in RON testing, but with a preheated fuel mixture, a higher engine speed, and variable ignition timing to further stress the fuel's knock resistance. Depending on the composition of the fuel, the MON of a modern gasoline will be about 8 to 10 points lower than the RON. Normally, fuel specifications require both a minimum RON and a minimum MON.

Anti-Knock Index (AKI), Road Octane Number (RdON), Pump Octane Number (PON), or (R+M)/2.
In most countries, including all of those of Europe, and Australia, the "headline" octane rating shown on the pump is the RON - but in the United States and Canada the headline number is the average of the RON and the MON, called the Anti-Knock Index (AKI).

Because of the 8 to 10 point difference noted above, the octane rating shown in the United States is 4 to 5 points lower than the rating shown elsewhere in the world for the same fuel. '''For example, 87 AKI octane fuel, the "regular" gasoline in the US and Canada, is 91–92 RON in Europe. 93 AKI octane fuel, the "premium" gasoline, is 97-98 RON in Europe.'''

However most European pumps deliver 95 RON as "EuroSuper" or "EuroPremium" (equivalent to 90–91 AKI). In Germany, Great Britain and some other countries, 98 RON as "SuperPlus" (93-94 AKI) is available almost everywhere. Even 100 RON (95-96 AKI), is widely sold, and BP sell 102 RON, marketed as "BP Ultimate 102", at some very select few filling stations.

What does all this mean to me?
The manual states that 91 RON gasoline should be used. As we already learned, North America uses the (R+M)/2 method, and that MON is 8-10 points lower than RON.

SO:

91 RON + 83 MON = 174

174/2 = 87

'''So if you are in North America, regular 87 is all that is required. Use of a higher octane does not provide more power (unless the engine is designed for it, which the 500 is not.) It does not "burn cleaner" or anything like that. In fact, it burns slower, and achieves a less complete burn, which can lead to carbon build up in the cylinder/valves.'''

If you are in Europe, Japan or pretty much anywhere else in the world OTHER than the US or Canada, look for the 91 RON.

Here [1] is a very helpful article that has a more thorough explanation of "Knocking" and why you don't need high-octane fuels. [Link was broken, avoiding replacing since I don't know what stance the original article took]