I change my spark plugs whenever I change my oil (2 to 3 times a year depending on how I'm driving). The longest they'll stay in there is every other oil change. Not a whole lot of point in using iridium if I'm not leaving them in there for two years straight... Not a lot of point i using platinum either. I usually run NGK's copper plugs these days and I haven't noticed any change in performance or mileage in any of my cars (I run double platinum in my wife's car because unless she reminds me I don't service her car regularly).
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