is there an additive I can use to stop a gasket leak, cost $700 to get repaired

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I was told by the Hyundai repair shop that I had a gasket leak and the cost to repair was over $700. I did see an additive that can be added to engine an seals leak for up to five years. Is this true. The problem occurs at all times but I don't see oil under the car but I smell after car shut off.