Just spit on it. Just kidding. Use whatever oil you have in your cleaning kit. I have been using Lucas Gun Oil recently and have been impressed how well it stays in place plus it has a low odor.
Whatever you have on hand that you put in your car. I have been told by numerous gun smiths that it is all they use. If you really think about it, an internal combustion engine works way harder than a firearm. If they were to take motor oil and turn it into one of those expensive "specially formulated for firearms" oils, they would have to break it down some.