I've found the best way to apply the stick-type compounds is to lightly sand the surface of the leather, creating a fine suede-like finish. Then just use the compound like a crayon. You don't need a lot of compound - in fact, if you can't see and feel the texture of the leather, you've got too much compound.
When it gets shiny or darkened from metal, scrape off the compound and re-apply.
No heat, no oil, no Goop.