The Rattler,
I would buy a copy of
Understanding Wood Finishing by Bob Flexner. It's about how to select the best finishes for your purposes.
On page 84 of that edition I linked to is a table titled
Guide to "Oil" Finishes. It rates both raw and boiled linseed oil protection as
Poor. This is because of the ability of linseed oil to attract and hold moisture. It will take on a watermark in a heartbeat, and it seems to me we've had at least one member reporting a BLO stock finish ruined by rain exposure (white watermark streaks). This is a very traditional finish, but it's probably been around as long as tung nut oil (a.k.a, China wood oil), which dates back to B.C. in China, and certainly since long before modern understanding of finish chemistry.
Pure tung oil (as opposed to a mix of tung oil with a resin heated to form a varnish, or a mix of a finished varnish with tung oil) is also rated
Poor for protection by Flexner, except when it has been built up with at least five or more coats with sanding between coats. Some people apply as many as a dozen coats, which takes patience, since each coat needs five days to a week to dry. Flexner comments that it looks dull and blotchy until you get five or six coats. It is one of the harder oil finishes to apply, and once applied, it is still thin and flexible enough that it doesn't provide the best protection, though some feel it looks best. It is still permeable to moisture over time, so it won't help with wood swelling in humidity, either.
The best protection, according to Flexner, is from polymerized oils. In this case "polymerized" does not mean plastic added. It just means the oil has been heated to 500° in an inert atmosphere to cause partial cross-linking (total cross-linking results in a solid, which is what happens when an oil dries) between some of the monomers and natural polymers in the oil to increase the average polymer molecular length. This also means the oxygen that causes oils to set and dry takes less time to work, as it has fewer remaining molecules to cross-link to the dried solid state. Those larger molecules are harder to break down, make a thicker finish, are more resistant to permeation by moisture and are mechanically are tougher than pure oil. Examples he gives are Southern and Wells brand Polymerized Tung Oil and Birchwood Casey's gun stock finishing product, Tru Oil.
I suspect the hardest and best protective finish is the one military armorers used to used for match conversions of M14's. This was a single-component epoxy finish. The dried the unfinished stock wood out, then submerged it in the finish inside a capped length of iron water pipe. The pipe had an air fitting on it, and they then applied 100 psi of air pressure to force the epoxy deep into the wood. The pressure was relieved, the stock removed and all excess finish wiped of, and then it was dried in a low heat oven for a week to force the epoxy to finish cross-linking. This finish essentially stopped humdity-related warping. It also made the stock rather heavier, but very tough and durable. At that point it was like epoxy glass, except with wood fibers instead of glass fibers in the structure.
There are likely a number of other kinds of things you could do to improve protection. Short of the epoxy, simple submersion in Thompson's WaterSeal prior to final finishing should help. I once reinforced a badly dusting old poured concrete basement floor by hitting it with WaterSeal, followed by a second coat that was 1/2 WaterSeal and 1/2 oil based floor paint I intended to apply, followed by a third coat that was just the paint itself. I intentionally left a portion of the concrete untreated and painted it only. That portion lost its paint to chipping fast, while the treated portion looked new until the day I sold that house.
Given the mineral spirits base of the original WaterSeal formula, I expect it could be used the same way with a wood finish like polymerized oil or Watco Danish Oil and the like, and would also improve the wood protection. You'd just want to have a heated drying box to get all the deep solvent to come out.
Finally, I'll echo what Doyle said, that many "tung oil finishes" use that name just because the final product resembles an actual tung oil finish. The name is about the final appearance and not the ingredients themselves. Indeed, this is true of many "oil finishes". Watco Danish Oil, for example, is just polyurethane. Totally synthetic. But it looks like a Danish oil finish. I've used it on a couple of Garands, actually, and got very good finishes from it. That's a case where I needed to use a product that was available with different stain colors so I could get a mix that made birch look like walnut in order to make parts match in appearance. In that instance I did not try the WaterSeal trick, as I was concerned I could not control the final color when mixing and matching finishing products. Especially not when I was waiting a long time for the solvent to permeate out.