I had a 2001 Accent and ran everything from 5w20 to 10w30 and the oil weight didn't seem to have a noticeable effect on anything. The OEM filter does quiet it down nicely. Here in the Midwest the cheapest gas is 89 octane which contains 10% ethanol. I always got the best mileage out of our Accent when I ran the 87 octane gas. I usually saw a pretty consistent 2 mpg difference between the two. I just started using the OEM filters on our Elantra and it is the quietest it has ever been. I'd try the OEM filter and stick with 5w30. Gas was pretty cheap when I bought the Accent so I never really checked my mileage with the 5w20 but I can say it was just as quiet with the 5w20 and OEM filter as it was with 5w30.