Hylomar is a brand, like Permatex. They make RTV, gasket sealants, locktite, etc. etc. A lot of companies have to to them for OEM products, ie, Land Rover uses their products, such as for the oil pan you mentioned.
Use it anywhere you think you might want to take things apart without leaving gasket welded to the mating surfaces. Especially handy for valve covers, oil pans transfer case covers, diff housings, and a host of other applications. Never dries, always stays gooey.
Use it in conjunction with the gasket, not instead. Not for high pressure.
I think the equivalent stuff available on this side of the pond is a Permatex product. I use Aviation (form-a-gasket) sealant by permatex. It comes in a small plastic can with a brush for application. Does not dry and works well.
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