If your owner's manual specifies that you should run 91 Octane then you should run 91 or HIGHER octane. In today's computer controlled iginition / combustion systems there is really nothing to be gained by running higher octane than required unless you have modified your vehicle's engine and / or computer. If you continously run a lower grade of octane than required for you vehicle then you run the risk of poor perforance and premature decay / damage of your engine.
In my opinion you can occassionaly run mid-grade 89 octane as long as you follow up with 91/93 octane; but why??? The price incremental between grades 89 to 91 to 93 octane (at least in New Jersey) is prox $.10 per grade.
In summary... give your VW want it wants and it will treat you well for a long time.
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