Additive to seal leaks can cause long-term damage to the car... In general replacing that manifold seal is a fairly simple task, but first step is to clean the oil up and make sure that bolt is tight / tighten it a very small amount. Then drive around for a while and see if the oil comes back. No need to think about it again if it doesn't come back or only leaks a small amount, unless of course you have other symptoms and then there's a different approach.
