2010 Hyundai Elantra manual states a 3750 mile OCI with severe service driving - but the manual doesn't state severe service OCI is based on conventional or synthetic ? I'd say 75% of my driving is severe service (suburban driving) and I just bought Quaker State Ultimate Durability 5w20 to try for my Elantra (4200 miles on it to date) . * How many miles should I shoot for as an OCI - 3750 or take it out longer due to synthetic Quaker State Ultimate Durability ?