Right now I have been using regular conventional, Pennzoil YB and Castrol GTX. I have enough GTX for 4 more oil changes, after that I am planning to switch to synthetic. I have a 2004 Hyundai Elantra with 48K. Which fully synthetic 5w-30 motor oil do you guys suggest? Plus, will I notice any performance gain (like smoothness, higher MPG and ect.)? Thanks!