Perhaps the simplest way to pin this question down is that NO Major firearms mfg. uses or recomends any Motor oil synthetic or otherwise. Nor does the Military use same. There are a number of synthetics, etc. that are used by both and most have MSN #'s. Mobil does not recomend its syn oil for use on firearms if you care to conctact them. FWIW, personally have settled on TW25B grease, actually a "creme" {Used in-house by Glock, *&*, FN USA, Sig, etc.} and Militec-1 syn oil a ca. 10 wt. that is actualy a metal treatment that is reactive to the heat produced by the pistol. Even when "wet" the stuff does not attract &/or hold firing "soot", etc. Tetra grease is also a "stand-by" for the newer lubes. Good info @:
http://www.mil-comm.com {TW25B} usually best $ @
http://www.bestdefense.com http://www.militec-1.com and
http://www.tetraproducts.com