I use the Superlube spray dry on all FCGs, regardless of the weapon. From A5 Brownings to Valmets...However, I do grease the rails/oprod/etc with synthetic grease. My feelings are that the ol M14 needs a smidgen of stuff to slide on. The drawback is that grease is a sand magnet. I use synthetics because their viscosity characteristics do not change nearly as much in the rain, heat, or cold.
This dry lube also has excellent below zero properties, I have actually used it on boltguns and semi autos at better than 10 below with superb results. Unfortunately it does not keep your toes warm.....GI3
Dry Slide is a great product as well, even Harley cables love it.