I have some discolorization on my bi-tone's slide that doesn't look like rust, but looks kinda odd. Its mostly in the recesses that aid in gripping the slide to cycle it by hand. The thing is, its not rust colored, and wont rub off. Its more like the slide color but slightly darker. As far as maintenece goes on this gun, I wipe it down with oil at the least, twice a week. Any ideas?