I read several different threasds about 308 spark plugs. However, I don't get too much concensus: Some people use NGK BP6ES, or NGK BPR7EIX. What's the difference between the 2? Do they need adjustment to fit correctly in the 308? What is the added value of iridium type? Why people would change from the Champion of Bosch used OEM? Do the recommandations differ if carb or injected car?