Back in the day, it was common to use paint. Those that used axes every day, esp in the NW, were coming into contact with wet conditions almost daily. From what I have gathered, read, spoke to, etc, the preferred method wasn't paint(too expensive over time), but rather oil. What exactly this oil was/is, I am not sure. Early refined oil from crude is what I can gather. Other things I have heard/read is pig or any other animal fat, and also bacon grease, which I know works 100%, because I use that on my cast iron skillets at camp when not in use. I suspect those guys in those camps used whatever they could at the time due to where they were.
Today, there are so many options I am not sure if you can go wrong. Myself, use this - this is me, take it for what you will.
WD40 for the most part. Now, WD40 is not a permanent solution. You will have to reapply. I do so about every 3 months, depending on conditions. I have mine in the basement, so that is different than in a garage. Different aspect will call for different applications.
One I have been on lately, especially for those axes I know wont be used or moved, is regular linseed oil. If you apply it to the axe head, it will coat it nicely in what I call rednect clear coat - but it will take some time. It will take probably 3 days to properly dry, depending on conditions. I really like this, as you wont have to revisit your axes like you will with WD40. However, I enjoy revisiting the collection and going thru and wiping them down. To each their own.
Bees wax, stp, tung oil, motor oil, etc. I have probably heard of em, all, and tried most, but not all. Go with what you think works, what you have, can afford, etc. The big thing is, don't leave that axe alone. It's old, been threw who knows what, seen more than we can imagine, so it deserves some love. So love it. And often.
Hope this helps!
Mike