WD-40 is not patented, to make it MORE secure against copycats!
Sounds backwards, but it's a smart tactic. If you don't know, WD-40 is a Water-Displacing Spray. When Norm Lansen invented it in 1953, he had tried 39 other formulas for his compound. The 40th was the winner, and so, Water Displacement, 40th Formula became WD-40.
It's an insanely popular lubrication product, and has virtually no competitors. The reason for this is that it's hard to replicate. Part of this difficulty lies in that WD-40 has not been patented. If they were to patent the formula, they would have to disclose the ingredients. By not doing it, they keep them as a trade secret and avoid having their competitors replicate it!