This from wikipedia: "Formulation[edit]
WD-40's formula is a trade secret. The product was not patented in 1953 to avoid disclosing the details of its composition; the window of opportunity for patenting the product has long since closed.[4][7] WD-40's main ingredients as supplied in aerosol cans, according to U.S. Material Safety Data Sheet information, are:

50% "aliphatic hydrocarbons". The manufacturer's website specifically claims that this fraction in the current formulation cannot be accurately referred to as Stoddard solvent, a similar mixture of hydrocarbons.[8]
<25% petroleum base oil, presumably a mineral oil or light lubricating oil.
12-18% low vapor pressure aliphatic hydrocarbon, to reduce the viscosity for use in aerosols. This fraction evaporates during application.
2-3% carbon dioxide, presumably as a propellant, is now used instead of liquefied petroleum gas to reduce WD-40's considerable flammability.
<10% inert ingredients.
The German version of the mandatory EU safety sheet lists the following safety-relevant ingredients:

60–80% heavy naphtha (a petroleum product used e.g. in wick type cigarette lighters), hydrogen treated
1–5% carbon dioxide
It further lists flammability and effects to the human skin when repeatedly exposed to WD-40 as risks when using WD-40. Nitrile rubber gloves and safety glasses should be used. (Ordinary rubber is ruined by repeated exposure to petroleum products.) Water is unsuitable for extinguishing burning WD-40.

Wired published an article giving the result of gas chromatography and mass spectroscopy, claiming that its ingredients also make it resistant to freezing.[9]"