As the title mentions, just wanted to ask fellow Subaru fans and anyone with mechanical knowledge their thoughts on the spark plug replacement as listed for my Subaru Legacy 3.6L and probably all EZ36 engines. Most Iridium plugs are bought for their longevity and the Subaru recommended plug on NGK's website lists a longevity of 100k miles, but Subaru still has a replacement interval of 60k miles.

Does anyone know if that 60k is absolutely necessary or if they're being incredibly conservative on their estimate for some reason? Or has anyone as mechanic or otherwise examined plugs coming out of these engines to see if they're actually at the end of their life or if they've got plenty of material left on them?