In a wet climate, you better believe I will use a light coat of light grease on certain parts of an M4/M16 or AR15. Even more so if one is going to be doing a lot of full auto or rapid semi-auto fire.
Light coat of oil is okay for plinking on the range, for heavy duty use you need more than oil that cooks off. Of course its a different story in the sandbox. many will disagree, and many will agree. To each his own poison.