I can tell you that if you take an antique pistol and polish the brass on it you will have just lowered the value of the gun. The brass on a gun should be cleaned with soap and water and then oiled the same as the rest of the metal parts. The patina that forms on the brass is part of its history. Keep it clean and lightly oiled and let the patina grow.
The military still expects you to present yourself with your brass and steel polished and shiny but not your gun. It is presented clean, lightly oiled and ready for use. If the weapon is presented completely clean and free of oil they will call you on it. If it is "wet" with oil you will be called on it. If the gun is meant to be a perfectly preserved show piece then it can be lacquered and put behind glass. The brass will still gain a patina and it will be left to do so.