From GrantCunningham.com -
One product that scores pretty well in corrosion testing is also readily available and dirt cheap. It also has good migration, a good boundary lubrication package, is the right weight (thickness) for general firearms use, doesn't oxidize over long periods of storage, and is compatible with a wide range of metals and plastics. In addition, it is recommended by at least one real degreed firearms engineer! Just what is this miracle elixir??
Dexron-type Automatic Transmission Fluid (ATF). That's right, plain ol' ATF. The kind you get at every gas station, auto parts store, and even most convenience stores. Synthetic or regular, either will work just fine. (ATF does have a slight odor to it. If you find that objectionable, a decent alternative that is still readily available is "NyOil." Check your local auto parts store, in the aisle where they keep the miscellaneous lubricants and additives.)