My owner's manual calls for 75W-85 (API GL-4) MTF but I just flushed that and put Shell Spirax S 75W-90 (API GL-5, MT-1, pour point -45C) synthetic I got from my brother's supply. The transmission was very stiff (until warmed up) on cold winter days with the OEM stuff. Could it have been dino oil? Is 75W-85 readily available in stores? The Spirax did great when I put it in my former '05 Corolla. Shifting when cold in the winter was just like in the summer and fast idle speed almost gained 200 RPM. I hope to improve the cold shifting of my Sonata the same. So far in summer temps, the change is almost unnoticeable.