I've been a data nerd on this stuff because the farmer's coop I belong to and COSTCO both 1-1.5 mi from home have good options and prices. Both have the "top tier" seal. The farmer's coop has the E-85, 10% ethanol, and all petroleum. They have 87, 91, 93 and special racing fuel. COSTCO has 87 and 93 octane. For my Outback it really did do slightly better with pure gasoline but it cost more to get better fuel economy - a no payoff. At best the pure gasoline was only worth it if I was heading to cabin with roof box and bikes on hitch rack. For my 4Runner a few times higher octane stopped ping. So did doing the scheduled tuneup. The VW is barely broken in but as said, I found a test that showed that turbo engine did have more power on dynamometer with higher octane. This makes sense knowing you can blow on the fire. I'm certain a test graphing output from idle to redline included blowing on the fire. I'll do same nerd/accountant thing with the VW now. Consider similar weather and wind for a cabin trip and different grades of fuel. I do think some of the advertising for that top tier detergent grade is a stretch or is nonsense if you consider the carbon build up issue and engine design. My understanding is all the direct injected engines without additional injector before intake valves are prone to the problem. I digress but also be careful about disinformation or wrong info on some engines. Some have newer rev/gen versions with fuel hitting the intake valves. For the carbon build up issue all my reading says it's blow back and contents of the EGR/PCV circuit that make the mess.