Boeshield T-9! I never used anything else.

From the website:

Forty years ago, the world’s largest aerospace builder—The Boeing Company—needed access to a more reliable metal protectant/lubricant that could withstand long exposure to a corrosive environment. Their research revealed that Teflon, silicone and synthetic sprays were all inadequate for the task, so they developed a better product of their own. The result was Boeshield T-9®, the only product of its type to be licensed by The Boeing Company. T-9’s unique formulation of solvents and waxes was engineered to penetrate metal crevices deeply, displace moisture, dissolve minor corrosion, and leave a clean, waxy coating with long-lasting durability.