When I shot on teams in the Corp's we would use grease on the bolt carrier to prevent the CLP smoke/ burn off from getting in our eyes during a match. But for duty use, it was/ is CLP only.
I got an AR in 80' an always oiled it...I noticed after several rounds all the oil was burned off anyway so it was running dry. In 82' my Army unit had Capt. Jesus Christ Dominque Rodriguez in charge, a Nam Ranger an we always kept the M16A2 bone dry. In fact after cleaning we ran them through hot water an dried before the Armourer would take them. One reason for this it that it was very dirty an sandy in GA. Oil an grease attract sand an dirt an that stick to the parts on the rifle an cause problems. Big debate on this continues on all the forums, I read a few places but can't remember the guys name, suppose to be the head guru of these rifles who said that a dry AR should run over a 1000 rounds without any problems, oiled should run over 1500 rounds....this is without cleaning.
Have ran them dry, lightly oiled, and dripping wet. Used various and assorted greases and lubes. Have had the action full of sand and squeaky clean, and they still ran. I use issue CLP mostly as I have about a quart left, but I do like the Mil-Comm products.
Grease is not recommended on the AR platform just as grease is not recommended inside a motor. Oil allows the contaminants to flow away from friction areas. Grease causes the contaminants to stay in place and act as suspended abrasive damaging tolerance areas of the firearm.
Running the weapon dry should also be compared to running an engine with out oil. It may run for awhile, but what damage is occurring to the tolerance areas as these areas heat up and make contact.
Military weapon maintenance has been shown for sometime to be improper and designed mainly to give troops something to do and to save the armorer time when weapons are turned in. Failures of the M-16 weapons system and failure of parts have been linked to excessive cleaning and improper lubrication.
Gun oils and engine oils have been engineered for different purposes and gun lubricants should only used for the optimum life and efficiency of the weapon.
I use Slip 2000 based on it's performance during extended testing with EAG Tactical and from my own experience with the lube during classes and during extended storage.
Here is the article on the BCM Mid-Length that Pat Rogers has successfully run for an excess of 40,000 rounds using only Slip 2000.