The major importance about the API service designation is that there -be- one, at least to the "Sx" designation called for in your owner's manual. A "Cx" designation applies for diesel engine use, and has no intrinsic advantage for use with a gasoline engine. Ironically, my 2003 Hyundai 2.7 liter V6 calls for API service SG or SH... (Is South Korea on the same timeline as the rest of the industrialized world?)
-Ray Haeffele