Oak or for that matter almost any wood finished bright that is not protected (garaged) will degrade as a direct function of the amout of UV as Julie stated. If your truck is going to be outside predominently and you want it finished bright, you are into an annual light sanding and recoating-2 coats/year. You are dealing with what I call on my wooden boat "horizontal varnish" which is the most difficult to hold finish on. I would recommend a minimum of 6 coats (I use 8-10) of good spar varnish or a product sold for marine use called Cetol.