replaced my stock plugs at 19,500 miles with CR9EIX (Iridium).
I changed my plugs out at 23000 and they looked just fine.. could have gone another 20K I am sure.. I would also replace with OEM plugs.. I have heard more than once guys going to Iridium plugs having mileage issues.. no idea why but not uncommon.. really do not have a good reason why they would hurt mileageI just replaced my stock plugs at 19,500 miles with CR9EIX (Iridium). Mainly cause that's all the stealership had in stock and I was desperate. Oh boy do I like them though!
Of course when you consider that my stock plugs had the electrode worn out to even with the threadbody, well, they were due.