If you can hide the nails above the reveal of the siding, use a galvanized 6 or 8P nail, if you must face nail, which is sometimes the only way you can get things to hold, then go with a galvanized finished nail. If you've got wood siding from the 20's it's liable to be very seasoned (meaning hard as hell) and predilling maybe in order to avoid bending the nails instead of driving them. Good luck.
All you need is ignorance and confidence and success is assured. - Mark Twain