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Silicone is a no-go for gasoline applications. The best material I have found is Nitrile rubber sheets (aka Buna-N, NBR, etc.) and it is approved for gasoline and many other oils and fuels. I laser cut it to make fuel pump diaphragms for mechanical fuel pumps in my older classic cars. You can get plain sheets or Nylon cloth infused sheets if you need extra strength. I've bought from ebay, McMaster-Carr, Amazon and Grainger in the past. Plain Nitrile should be fine for your fuel tank sender gasket.