I use a light coating on the lugs, especially where I can see the "shiny" areas from wear at the engagement points. I've used all sorts of oils, but the film lasts only a couple mags before it is worn away. However...after using a bit of grease, I find it stays right through the whole range session, and is still there when I take the bolt assy. out to clean. You can tell it has been smoothed over a bit from contact with the engagement surface, but the lugs are still coated. I also use it on the wear points all over the bolt and carrier assy. Works great. In fact, I've become such a convert that I use it on all my weapons, be they FAL's, M14's or AR's.
I use the white Tetra gun grease, and although I'm not subjected to the sandstorms of Iraq, I haven't found there to be more attraction to dirt vis-a-vis oil.
That's my .02. YMMV.