
It looks like the seal for the MAP sensor.
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And at least on the newer 3800's, they just use the rubber thermostat gasket. Won't hurt to use both I suppose. It will only leak with just the rubber one if you don't get all the corrosion out of the groove.
My personal preference is to stick with the hottest possible thermostat. If you are modifying the engine to be racier, or maybe you have some temperature-related detonation/knock (and are using sufficient octane), then maybe go with a cooler thermostat. But a 195 degree thermostat will give you better MPG