The darkening effect may be taking away from the metallic in your perception. Don't be mistaken with darkening as Mike Phillips has pointed out; darkening is actually bringing out the depth of color which can sometimes be perceived as a bad thing, until you see a 50/50 of a dark paint with/without darkening. I have layered 845 over DGPS on my pewter metallic S10 and haven't noticed any lose of shine in the metallic. I love 845 because it buffs off so easily, and leaves the surface slicker, longer IMO. Try parking the vehicle in question in the opposite direction and see if the light reflects differently; a lot of times metallics in paint are somewhat directional because of the relation in which they were sprayed on. It all comes down to your perception of how a product works on your vehicles. Like Mike also says, "When you find something you like/works for you use it often."(Probably not an exact quote)
What color is the truck, btw?
