0/5W-20 in early Theta II engines. 7~8K OCIs and the oil would be so fuel diluted that it would damage the bearings. Since then, 5W-30 has become the default recommendation for 2.4 GDI Thetat II and 5W-40 in the 2.0 T-GDI. Hyundai never admitted to the fuel dilution issue and instead had other stories to explain the engine failures. They had to lie because they used 5W-20 for the EPA fuel mileage test cycle. They were even sued for the stated fuel mileage not matching the Monroney sticker, not even close because once you run a different oil, you get a different MPG result. Dealers started getting service bulletins asking them to run the more viscous oils in these engines. Many just defaulted to 5W-30 for everything, which kind of left the 2.0 T-GDI owners to hang out to dry. Thank God that there aren't that many around anymore as Hyundai has moved on to newer engines.
That being said, I run 0W-20 in our 2.5 MPI/GDI Hyundai Sonata. But this engine does not dilute fuel, at least not at the rate that the older GDI Theta II did. I run 5W-30 in my fuel diluting 2.4 GDI engine in the Santa Fe Sport. The 2.5 Smartstream engine with 0W-20 is very fuel-efficient. It beats the crap out of the older 2.4 GDI engine. That engine can't deal with fuel edited 0/5W-20 oils. When dealing with fuel dilution, it's good to have a safety net.