I like Clenzoil. Some other common suggestions are Breakfree, M-pro 7, and Remoil. Hoppe's #9 may be used, but I wouldn't soak a nickel gun in Hoppe's for an extended period of time. Clean what you need to clean, wipe it off, then apply a good oil or preservative.
Cleaning an old gun is not much different than cleaning a new one, just don't be overly aggressive. Less is more when attempting to preserve the appeal and value of antiques.