*I still hold that I did see a slight increase in mileage from the "thinner" 5W30 synthetics in my Hyundai 2.4L GDI - but I can't count out either that Fall / Winter gas change over has something to do with this . I will also add that with a 5W20 synthetic - the Hyundai 2.4L Theta II runs like a "scalded dog" and was noticeably more responsive with throttle increases . Alas , the engine became quite a bit louder with 5W20 synthetic in it as well making me believe that a 20 weight oil just is not up to the task in this engine . Factor in fuel dilution , resulting in sheering and then questions of film strength and no wonder Hyundai / Kia dealers no longer use 20 weight oils in the 2.4L Theta II GDI engines .Actually, I do tend to agree with you on this. When I bought car in Dec 2020, the first oil change I did was with M1 5w30 ESP which is almost a w40. Now I've had Kirkland 5w30 for the last 5 months and 3-4k and I prefer the way the engine feels. I have yet to do a UOA but will in about a month. But the thing is there is no way youre going to see 1-2 mpg gains from such a miniscule difference in viscosity. You're better off using a high quality 5w20 in the winter unless short tripping.