my 01 z 28 came with denso iridium spark plugs from the factory? i did my first spark plug change today(54,000 miles) and it had denso plugs in it. (ptj16r15 12563707) these are the ones i took out of it. i went to auto zone and purchased densos not knowing what was in there(purchased before i started the change) . i assumed it was ac delco. the ones they gave me at auto zone were different(it16 5325) what the hell is up