Almost every competition shooter I've known of used a synthetic motor oil on their firearms for lube. Their firearms get hammered with rapid fire the norm.I’ve run Mobile One 5w-30 synthetic on pistols and AR-15 bcg’s for years.
I’ve been using Valvoline synthetic on mine for almost 20 years in competition and casual shooting/hunting without any rust issues or heat related problems. Never gets gummy, never gets thick in cold weather, never rusts.