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"Need"? Probably not. I change on 50k mile intervals and visually they look fine when removed. But they're inexpensive and I have noted a small, but notable different in lower-RPM, high-load operational smoothness after changing them, so I will continue to do so. In addition since this is a steel-into-aluminum interface (the head) leaving them in the engine for an extraordinary length of time runs a risk of galvanic corrosion that I find unacceptable, since a damaged head (e.g. the plug hole threads, if it happens) means lots of money to fix.
Mazda's "branded" plugs are ridiculously expensive, incidentally, should you choose to use them. Note that a fairly serious degree of caution and a torque wrench are absolutely required to do this job; if you cross-thread or strip the head you're in a world of hurt.
I haven't had trouble with the NGKs -- Bosch also makes good stuff.
Last edited by tickerguy; 06-20-2018 at 10:48 AM.