I used the Lightspeed supplied W27-EMR plugs the first year, iridium IK-27's the second year, and now have a set of NGK's installed. I've noticed no difference in operation. Only difference for me was the price. I still have the iridium plugs, but will probably just buy NGK's each year and throw the old ones away - they're only about 2 bucks each. The advantage of the iridium is that the electrodes won't burn away as fast and should last 100K miles in a car..... not something we worry about in our airplanes.