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If you pay someone to put plugs in, I'd get iridium so you can go the longest interval before having to pay a mechanic to replace them again.
However, if you're going to sell the car within the next 5 years, I'd just go copper.
You will get a CEL if one of the plugs is causing a misfire. Otherwise, the plugs won't have anything to do with MPG.
I'm not aware of any adjustment that can be done to intake/exhaust valves to get better fuel economy. My assumption is anything other than stock would get worse fuel economy. Perhaps it could be adjusted for top end power, but that would certainly hurt fuel economy.
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