@spoofy My opinion is the quality of gas is the most important thing for the GV70 and almost any car's engine unless the engine "requires" premium instead of "recommends" premium. That was the same advice given to me by the Genesis tech at my service department, he fills his GV80 with 87 or 89 octane.
As stated above Costco, along with many other places, have quality gas. Normally if the gas station is a regional and national brand, you are safe. Places that
normally have poor quality gas are privately owned and always have the cheapest gas to try and tempt people to fill up there.
I normally fill up at BP and use their 89 octane but have also used 87 octane from a regional station. I'm guessing I might be giving up a small amount of horsepower but I've never noticed a lack of power when in Sport+ mode and doing a hard acceleration. I've had no other issues and I continue to get the estimated MPG on the window sticker or above based on my Fuelly app. BP is normally about $0.05/gallon more than surrounding stations, but they have a customer loyalty program that requires me to only type in my phone number and that program erases the price difference.