I'm replacing (long overdue) plugs in my daughter's 2011 Kia Soul 1.6L, I believe the OEM plugs are copper, Champions I believe but I haven't pulled them yet to be sure. Anyway, assuming the OEM spec is indeed copper, should I stick to them or is there a performance or mpg benefit to going to Iridium or platinum plugs? I'm inclined to stick with OEM spec unless there's a reason not to. I'm not real worried about the life span, I can live with the 30k interval of copper plugs if need be, I'm more concerned about function and having all the parts play nicely together.