I would never use slicone oil - even externally. Too much risk of it getting between metal to metal contact areas and causing galling. Don't ask me how I know
. BSA are correct.
Ballistol, used sparingl, is a much better choice for use pretty much all over. I wouldn't use it (or anything else not recommended by the manufacturers) on the internals of a PCP, though.
Personally I would not let anyone else service my guns a) on price and b) because I wouldn't trust anyone else with my pride and joy. Depends what type/make of gun, of course.