Re: how does oil finish age?
You are talking about very different finishes.
Natural oils, such pure Tung Oil, or Danish oil has to be replenished over time. Oils containing polymers such as Tru Oil (gunstock) or Tung Oil Finish (Minwax) dry harder, and will generally last years before needing to be replenished.
Natural oils will not seal or protect the wood from moisture as well as the polymer finishes.
Birchwood Casey Tru Oil is an easy finish to apply, care for, and feels great especially if you final finish with the gunstock wax (ala EBMM).