As long as it doesn't include graphite, automotive moly grease should work fine. It's not what the tech manual calls for, but it should be fine for civilian use. The tech manual calls for a particular grade of molybdenum disulfide grease that has more moly and less grease, making it better able to resist high temperatures and (more important) high torque situations under high temps.
Graphite is NOT a metal-it's a form of carbon. Graphite is a problem because it tends to absorb moisture and form an electrolyte, and because it's abrasive. Using graphite lubricants can wear on the anodized coating and expose unprotected aluminum to the elements. The risk of galvanic corrosion comes from the dissimilar metals aluminum (the upper receiver) and steel (the barrel and extension). Graphite, as I said, can soak up atmospheric moisture and hold it against anything it's applied to-which would not only provide the electrolyte for the corrosive action, but would HOLD IT IN PLACE to make the corrosion more concentrated and thus more damaging.