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When I upgraded mine 1.4 years ago, my mileage difference was 40k, my cost was 10k. All numbers were approximate. You really does not need to change the plugs at 60k, they will last at least 100k, I just recently changed mine at 96k, and still look pretty good.

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I never look back, my only concern on 460 is that it consumes engine oil like crazy; but according to my search on this site, that is normal, my 430 never consume any oil in part I believe it use a thicker oil. 460 uses super lite oil, 0w-20.