Dude, I changed my plugs on Saturday and found the same thing. Two different kinds, one kind on the left and another on the right sides. Yeah, could someone explain why the heck Ford would do this. Mind you neither of the two types of plugs were the ones that were recommended for the car. Heck it's right there on the label under the hood, people!! They can't even follow their own instructions.
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1996 Mustang GTS