The only thing WD-40 is good for is shining up dirty ATV or dirt bike plastic. It's a crappy lubricant, and an even worse metal protector - unless you enjoy applying it every week in a damp environment. Water washes the stuff right off!
LPS-3 is on the completely opposite end of the spectrum as far as metal preservation goes. Top of the line - you won't find anything better. It's also VERY expensive.
There are several in between, like Cosmoline, and Amsoil Heavy Duty Metal Protector. Cosmoline is like watered down LPS-3, but since the stuff isn't going to sit for 2 years, it's a cheaper alternative, & works GREAT without any re-application.
I just picked up a new boiler tube brush - I'm ready to clean .... but I might just let this thing run all summer long for DHW. We'll see - have to see which is more economical first.