Wheel bearing grease on the BCG rails is awesome. I like oil in the BCG/on the bolt but grease on the rails is works well for high wear areas.. In CA I rarely have to worry about temps that effect it much. If I am taking my AR out with me when I coyote call for a friend in the mountains where it drops in the 30's I will just run oil on the rails.
10mm.......the original .40