A little internet sleuthing has me feeling pretty sharp eyed....
Danish Oil and Tru Oil are both hard drying since they have resin varnish in them.
- Danish Oil is a equal mix of 1 part varnish, 1 part boiled linseed oil, 1 part thinner. It is considered a penetrating oil
- Birchwood-Casey Tru Oil is 3 parts varnish, 1 part linseed oil, 4 parts thinner, so it dries faster and harder of the two. It is not a penetrating oil. It's "on label" use is for gun stocks
- Tung Oil contains no resin, it's just tung oil that we thin. it's a drying oil, but it doesn't harden
G&L uses Tru Oil on their "gun oil" necks.. Fender Custom Shop and Musikraft use danish oil on their oil-finished necks. Warmoth says, "All our necks are dipped in an oil based penetrating sealer which is compatible with virtually all secondary finishes. This provides enhanced stability; however, it is not adequate protection for playing."
As you mentioned, roasted swamp ash turns a dark walnut color when coated.