I found a 12 ft orchard ladder buried in a mess of lumber and plywood, outdoors, when I started to go through the stuff Dad had accumulated. From the style I estimate it is well over 50 years old. It must have been lying in that pile 10 years minimum, probably more.
It had far too many fine splinters to use, until I sanded the whole thing. I also put penetrating oil on the tension rods and got them tuned up snug. It's now an excellent ladder with no further treatment.
There must have been some kind of penetrating sealer in it originally for it to be free of dryrot after this neglect.
Then I bought a similar 14 ft ladder from an active cherry grower who was selling several. He stored his in a barn, but they didn't look much different from mine. He recommended sanding the splinters down then treating it with a couple of coats of his special recipe - used motor oil thinned with diesel. Since his ladders didn't look any better preserved than my neglected one, I haven't put anything on either one.
I agree that any treatment should be penetrating, not a coating.
But I far prefer to use my aluminum orchard ladder. It's lighter, stiffer, and no chance of splinters. The wood ones are mostly for guests who want to come up and pick some apples.