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+1 to the OEM spark plug.
I have tried many different plugs in my Jeep XJ including bosch +4, E3 and even a set of iridium plugs. The best all around performance and fuel economy was always with the OEM plug. FWIW I have tried indexing the spark plugs but have had no noticeable gains in power or economy. Although it was only in the real world not a dyno.
HTH
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