Loctite manufactures fairly diverse Adhesives, Sealants, Lubricants and more. They manufacture a product called Rust Extend, which I use on my pasture gates and trailers. I have used POR 15 in the past and Rust Extend is easier to apply and works just as well - IMHO And yes, for you Fluid Film enthusiasts, you can use it as a top coat once it is cured for that extra double protection.
Another obscure fact - Loctite and Permatex were part of the same company at one time. Ever wonder why some of the adhesives use the same Part Numbers. BE careful about using color for buying/use decision. Always check the product specifications. Many of the same color products have different cure rates, void fill abilities and most important, working temp ranges. Nothing worse then using a product that liquifies at operating temperature. Makes for a very bad day...... Message to take away, use the manufactures web sites to ensure you use the correct product. The Parts Counterman is not always the SME (Subject Matter Expert).
Since this all started with a question about block heaters, KATS's (Five Star Manufacturing) is a good choice. They have an external pad that uses adhesive to attach to the oil pan. I seem to recall through the years other RTV owners have used them and spoke positively in this forum. I've met the owners and they are RTV owners as well.