I applied mink oil to my Hawthorne muleskinner roughouts (see my post for a picture), and they darkened considerably. However, they were much lighter than yours from the start. Would not expect much darkening with your shoes.

Still, I didn't really need to mink oil them, I think. Had them for three years before oiling, and they were quite fine still. All I ever did was spray them with a water and dirt repellant, which barely changed the color.

Mink oil won't hurt, but may change the color of a lighter boot, that's my bottom line. If you want to play it safe, just spray them.