Wondered a bit at first myself about the wisdom of that, though I'm all for recycling used motor oil for things that other people here say are terrible ideas (like using it for bar chain oil). But couple things about this application that suggest it's not likely a problem - horses chewing on wood are bored horses who generally aren't being kept or fed properly. It's not natural behavior. And this is wall paneling that is next to impossible for a horse to chew on, it's not a wooden gate or fence. It's an effective sealer less nasty than creosote - though like creosote, it's better suited to outside applications where there's no contact with it. I mean, end of the day it's kind of pick your poison when it comes to treating wood, not a lot of anything is great. I probably would have used Thompson's, but it's far from harmless too - unless you're using redwood, cypress, or cedar that you can leave natural, treating wood to be water resistant generally isn't going to be good for anything that wants to gnaw on it.
