Used motor oil is supposed to be disposed of properly because it contains a whole bunch of really *nasty* chemicals including a number of vigorous carcinogens. Used to be sprayed as a dust treatment on country roads and farm lanes and horse arenas, sometimes followed by a spate of things dying and people contracting cancers. If it gets into the water table it can be pretty bad for you in wellwater, too.
If your chickens are walking around on bare spots in the litter on a floor that was used-motor-oil coated, they are absorbing some of it thru their skin. Worse, if one has put it on one's deck and then walks on it barefoot or sits down or whatever, you're absorbing some of those chemicals through YOUR skin.
Just cuz it doesn't kill you always or immediately doesn't mean it's not a bad idea.
Use thompson's water seal, or stain, or just prime and paint. Safer.
Pat