I have been using Royal Purple, full synthetic 20w50 on all my guns. Applied with a q-tip, or syringe, depending on the application. We also use it on all the range rental guns at the shop....good stuff.
Nothing wrong with RP products but not all synthetics are the same , My buddy picked up a qt of some cheap synthetic at the fuel station on his way to a class , got thru about 100 + rds and that stuff gummed up so bad it would not cycle even after more lube . So he created a stoppage in the class . After a good cleaning and relubed with Mobil 1 his BCM ran fine the rest of the day
i've used 15w-50Mobil 1 in the past.
As hot as guns get ( just like engines) I didnt see a need for a thin grade like 5w-30.
I like to run the BCG wet, but don't like it to drip like thinner oils sometimes do.
But after a while i went back to Hoppes. It simply smells better than the Mobil 1