*I have tested various 5W20 , 5W30 and 10W30 synthetic oils in my 2017 Hyundai Sonata 2.4L . With 5W 20 weight synthetic oils the 2.4L Theta II engine runs nice and free - but due to fuel dilution and potential rod bearing issues - the question of not enough film thickness is often discussed as these engines continue to fail , so no wonder Hyundai / Kia now have gone to 5W30 oils for more film thickness head room . So, next I have experimented with thick 5W30 synthetic oils which Hyundai / Kia have now turned to in this engine (up to cSt @ 100 degrees C 12.0) as well as thin 5W30 synthetic oils down to cSt @ 100 degrees C 9.8) . The verdict ? The best running 5W30 synthetic oils in my Hyundai 2.4L Theta II engine have a cSt @ 100 degrees C. of between 9.8 to 10.6 (PPPP 5W30 , Valvoline Advanced 5W30 and M1 5W30 EP) . When I use thicker 5W30 synthetic oils greater than cSt @ 100 degrees C. of 10.6 from 11.0 to 12.0 (QSUD 5W30 , M1 vanilla 5W30) , I notice the engine starts to strain more (especially when cold) and over all performance starts to suffer as if the engine is straining more to pump the thicker oils . Lastly , I have checked my gas mileage and approx. 1 to 2 MPG improvement with the thinner 5W30 synthetic oils with this engine . As with anything , variable do exist - I'm just throwing it out there that from my experience after almost 5 years with this engine the "Goldie Locks" sweet spot synthetic oil for this 2.4L GDI Theta IIGDI engine is to pick a 5W30 synthetic D1 / Gen 2 or 3 with a cSt @ 100 degrees C. of between 9.8 (PPPP 5W30) and 10.6 ... I myself feel the best compromise in this engine as far as protection cushion with performance considered over a 5K mile OCI are the Valvoline 5W30 Advanced / EP and the M1 5W30 EP with 10.5 and 10.6 cSt @ 100 degrees C. respectively .