When comparing new brake rotors for my vehicles, I notice that some rotors mention in their description/spec that: They have a rust preventative coating! I see this on many auto parts sites. For example, the AAP WearEver Rotors mention a rust preventative coating but, the WAGNER DO NOT mention this. I have used both rotor brands and notice no extra rust prevention while on the car...thay all rust the same! Is this just a light maching oil applied to the rotors after mfg'ing for storage while in the BOX???