It doesn't seem to wear away any faster than regular undercoating, but is easier to touch up when necessary. The problem with boiled linseed is that it does get hard, and inflexible--therefore, it tends to crack and have moisture get in behind to accelerate rust. the raw oil stays flexible and is actually aided somewhat by a good coat of road dust (back where I grew up, undercoating on the trucks consisted of a spraying with old motor oil and a couple miles of dusty road!) Inside rocker panels and other hidden areas, as long as the surface is dry when it is sprayed on, it lasts forever (or at least 37 years, in the case of the '64 Ford). You can buy the stuff by the gallon at paint stores or full-line hardware stores, real cheap. Shelf life is indefinite, but it helps to move to smaller containers, rather than a half-empty lage container, to keep down oxidation.

Best time to do it is hot dry weather, so you can let the vehicle sit in the sun for a day or two after cleaning and before spraying, to make sure it is really dry--hot metal also encourages full coverage in crevices.