When a spark plug burns out it is not enough to go off of unless a spark plug that went bad was looked at. A plug may go bad from running too lean from a injector that's partially blocked for example. This will cause the center electrode to get melted.
The plug may get oil fouled from oil that gets into the cylinder from the valve seals or the piston rings. I'd suggest saving the old plugs so that you can show it to a mechanic to pinpoint the issue. A certified technician, such as one from YourMechanice, will be able to assure a clean and lasting replacement of your spark plugs. Good luck!
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