There seems to be much confusion in the VW world about which octane gasoline should be used with air-cooled Volkswagen engines. Many people say 91 octane. Those people are technically right, as long as they are not in the US.
Volkswagen recommended 91 RON octane gasoline for use with their air-cooled engines. However, here is where the confusion sets in: The RON method of octane measurement is not used in the United States. In the US, a different method entirely is used, called the CLC method. The number that results from this method is the average of the RON octane number and the MON octane number, so (RON + MON)/2 = CLC octane number. This is the number that you will find printed on a yellow label on gas pumps in the United States.
91 RON octane is equivalent to 87 CLC octane. My source for this claim is the Volkswagen owner's manual for both the 1978 Bus, the 1974 Karmann Ghia, and the 1981 Vanagon.
Therefore, if you are buying premium gasoline in the US because you think you need it, you are mistaken and are wasting a lot of money. A stock VW engine with stock ignition timing, etc., will run without problems on regular 87 CLC octane gasoline in the US.
Octane has to do with the resistance of a fuel to preignition, that is, detonating in the combustion chamber before the spark plug fires. This event is destructive to cylinder heads and the VW air-cooled engines are especially vulnerable to damage caused by this "pinging." If you are experiencing pinging with a stock engine, there is most likely something wrong with your timing setings. Use your timing light to check your ignition timing persuant to my article on ignition timing. A fuel with a high octane number is more resistant to preignition than a fuel with a lower octane number. The only reason you would NEED high-octane fuel would be if you had a high-compression engine, which you don't if you have a stock VW air-cooled engine.
Some people like to use high-octane fuel because they feel it gives them better gas mileage or perhaps cooler engine temperatures. If you like to use high-octane gasoline, go ahead. It won't hurt anything except your wallet. However, this article is just so that you will not buy high-octane gasoline under the mistaken impression that it is necessary because VW recommended it. That's just not true.
November 6, 1997