The WD in WD40, stands for 'water displacement ' .
It was primarily used as a water displacement fluid, like on ignition systems, and as a metal surface corrosion preventative .
This varies by region and country, but here in the US, Kroil, is considered a top notch penetrant oil .
I've used the aerospace trade name product, Aero-Kroil, for about 30 years, at least at one time, it was the only penetrating oil authorized to be used on commercial aircraft .