Oh yeah. I was in the VW scene for a long while. I had a 2.0T 2010 jetta wolfsburg edition 6MT. It was sick. However, when you pop the fuel cap, just like my SI, it says PREMIUM (91) REQUIRED.I read on other Hyundai/Veloster forums that Hyundai actually came out a year or two ago and said that their 1.6T Gamma engine is more than fine to run at 87 however the power numbers they quote are obtained on 91 where the retardation isn't happening as knock is to a minimum. So 87 is the minimum but power suffers. How much not sure. I've been trying to find that article.
I came across a VW article where they said the same thing. Their 1.8T and GTI engines(2.0T) can run 87 fine however the power numbers on the car are obtained at 91. It makes you wonder about this for all other turbo engines on the lower end of the power spectrum.
So if this is correct it would bug me that I'm not getting the full potential of the engine by using 87. Would it be noticeable? Not sure.
I will say I'm on my third fuel up on my 2018 ES that I just bought and I felt quite a bit of more hesitation and less pull as I'm using 87. The dealership fills theirs to 91 so that's what was in my tank at new. And before you say placebo/imaginary, I hadn't even thought about fuel types until a day ago. I just slowly noticed it myself over the last week. Or I could be getting used to the acceleration lol. But it definitely hesitates more.
Fuel mileage has gotten better but now my engine is past it's break in (1200KMs) so that's to be expected. So can't speak to that. I'm going to fill to 91 tomorrow and see.
It's GDI it will not clean your valves just bye putting good fuel. You have to perform basically like top engine cleaning! every Hyundai dealership perform this as a maintenanceIf you use top tier gas they will keep the fuel system clean. Adding additional additives won't be needed and likely won't do much of anything to help with carbon buildup.