I use the Denso spark plugs...not the Iridium plugs but just the equivelant to the NGK V-Powers and they are cheaper and MUCH better quality than the NGK. I have been running mine for 60,000 km (37,500miles) and the "V" groove is still there. a couple years ago I ran the NGK V-powers and the "V" groove was gone in 5000 km (3125 miles).