OEM is NGK Iridium spark plugs, with a recommended change interval of 60K miles. Every other Japanese car I have seen using NGK Iridiums has a change interval of 100-120K. Subaru seems to be overly conservative with the maintenance recommendations (e.g OCI of 6K with 0w-20 Idemitsu full synthetic on a non-turbo engine). Is changing the plugs at 60K just a waste?