Nix the WD-40
Nix the WD-40. It is kerosene and soap, it will attract moisture and aid in the corrosion of the stay once the kerosene evaporates leaving only soap.
The issues are much more complex than a simple application of sheep's sweat (anhydrous lanolin) or bees wax etc.
Crevice corrosion and work hardening are the two killers of SS standing rigging. The bigger issue than possible moisture intrusion into a swage is work hardening which happens over time.
I used to have some well done swages cut in half. If the swage is correctly done water intrusion is not an issue, the swage cold forms the metal of the terminal and the metal of the stay into one solid piece. It is pretty impressive.
The process for old terminals splitting is different, it is caused by crevice corrosion and expansion inside the swage. The crevice corrosion allow the water to enter the swage, the corrosion expands the swage causing the crack that you see on the outside. The water did not just flow into there to begin with.
Remember Rust (corrosion) never sleeps.
Most people think rigging lasts forever. In reality depending on where in the country you have your boat, a SAFE lifetime for most rigging is between 5 and 15 years.
That 30 year old rig that looks fine, is not fine. Work hardening has significantly degraded the rig. Even just sitting at the dock, or heaven forbid being on the hard with the rig up!
Guy