Some engines can be damaged by running an oil that is too thin for cold start and warm-up running, especially with cold piston to bore clearances. For example there will be excessive piston and bore wear in the engine in my wife's Opel if anything less than a 10W30 or 10W40 is run in that engine, and similarly with my Triumph motorcycle which must run 10W40. There may be a reason why Hyundai specified 5W40, and often the 5W is more important than the 40.
I don't know why so many want to ad-lib with their cars. Hyundai is a big company which has built millions of cars and they aren't stupid. If the VT will run 0w30 or 0w40 or 5w30 or 5w40; don't you think they would put that in the handbook?
But it's your engine and not mine. Go ahead and run what your heart's desire.
========================================================================================================Mobil1 is a group III mineral-based oil and has been shown to cause sludge in this engine, especially if you go past 3000 miles.
The only group III oil I recommend is Pennzoil Ultra Platinum, as it is designed to protect against LSPI.
Ideally you want a group IV (PAO-based) or group V (ester-based) oil with the highest flashpoint possible.
I understand if you are a Mobil1 head from years past. I used to run it in non-GDI vehicles with no issues. However, it just isn't a good oil for this T-GDI engine.
Couple things wrong here in bold.I did a lot of research on which oil was what when I bought my Veloster turbo, because turbocharged engines will run hotter when running on boost, and therefore will cause more oxidation (thickening and sludge) in anything other than a genuine synthetic oil. I absolutely, 100% guarantee that way back in the dark ages, Mobil 1 was a Group IV synthetic oil, but when the Castrol case went through and it was possible to label mineral oil as synthetic, then Mobil 1 became mineral oil (Group III synthetic). It is mineral oil-based today. I found three genuine synthetic oils: Motul when it's labelled synthetic (rather than techno-synthesis), Amsoil Signature Series which is not readily available in Australia, and Australian-made Penrite Racing.
The main area where synthetic oils are significantly better is oxidation resistance, or the thickening of the oil and it turning to sludge. Even in modestly-powered, normally aspirated engines there is an advantage in running Group IV synthetic in order to minimise oxidation. The other advantage for all engines is better flow when cold which reduces engine wear. In Australia once you sort out the facts of oil base stocks, running Group IV synthetic is a non-decision. Mobil 1 can't be had in the required viscosity of 5W40 so we can rule that out. An equivalent would be Castrol Edge Group III synthetic, 5W40 5 litres costs $59.99. Motul X-cess 5W40 Group IV synthetic costs $75 for 5 litres, sot the cost differential between fake and real is small.