Sealants won't seal to oil. I would carefully clean the old gasket off with a ravor blade or gasket scraper etc but don't gouge the surface. A thin blade screwdriver also helps at times. Once the heavy/thick portion of the gasket material is off, you car usually get the rest off with a wire brush.
Then clean the surface with lacquer thinner or brake cleaner, then with rubbing alcohol.
You can then just put a thin layer of some type of sealer like RTV on both sides of the gasket and it will never leak. Yamabond or "Hondabond" will likely work although those are intended more for use between the cases where no gasket is used. Gray high temp RTV from an auto
parts store or home depot/lowes etc will definitely work but let it dry for 12 hours before putting oil in it. An FIPG sealer will also work if you happen to have any of that around.
This is killer sealer
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This is also killer but it has a short working time, meaning that it can dry too much before a part is assembled but this wouldn't be a problem for your app.
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