Run the octane the VW manual states. There also should be a sticker on the gas fill door. In my case, on my 2003 Beetle 2.0 it is premium 89 octane for U.S. Even in my new 2017 Golf Sportwagen 4Motion it states 87 octane. I alternate 89 and 87 because of the carbon that is generated burning 87 octane in VW engines. Be warned, if you always run 87 octane in any 2005 or newer VW, the EGR system will become plugged with oily carbon at around 150,000 miles. You will continually have issues with the Check Engine System alerting you to some issue. That rectification will require dis-assembly of the Intake and EGR Systems to painfully clean all passages with small wire brushes and Brake Cleaner. I had to do that on my 2 daughters 2005 Golf and 2006 Passat. They both ran 87 Octane till the 150,000 miles range, and then brought them to me to fix.
If you don’t plan on keeping your VW for more than say 80,000 miles, then the computer will compensate for the octane by reducing or increasing power as the knock sensor indicates. Your mileage maybe some what lower. With my octane alternating, I am getting 29 mpg in town and 38 mpg on sustained highway miles with the 1.8 TSI 4Motion and the DSG Transmission. I do use the paddle shifters a lot rather than mashing the Go pedal as much.