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The sealing lip is always made of a rubber or synthetic material. For oil seals with a rubber outer case (R, RST, GR, GRST), the rubber quality of the sealing lip and the outer case are the same.

The cost of new spark plugs can range from as low as $2 to as high as $20 per plug. Cheaper spark plugs are often made of basic materials and may not last as long as more expensive options. On the other hand, high-quality spark plugs are made of premium materials such as platinum or iridium, which can resist wear and corrosion better than cheaper materials.