Some time ago I remember my mother telling me that way back farmers used to take the old motor oil from their tractors and use it on thier barns. I guess as protectant/stain.
I experimented on a fence and it looked awfully nice! Smells for a day or two, but the "color" holds up over time. Then I did a 10 X 10 section of a 100yr old barn. Faded barn boards came alive again! Rich dark look is what you get.
I haven't myself, but I know a guy who used it on a small log storage structure that he built. Said it worked pretty good. I don't know about the long term effects from it. There are quite a few additives in oil and gasoline.
I recall my grandfather doing this about 45 years ago. He would collect used motor oil and add to it "red lead" powder. He once used green powder but red was his preference. His neighbors would come and help "paint" this mixture to the barn or shed or what buildings he was painting. This would protect the wood and beautify the buildings and this was done on an annual basis. You had to take precautions to not rub against the painted wood forever as it would color your clothes and was just about impossible to remove. You could also see this on cattle that used a painted post or corner as a rubbing device and they too were colored. Lead Powder is not used any longer as it is no longer available. There are still buildings in my area that have a red tint to them and are the results of the red lead and oil mixture that was applied that long ago.
OK, this is gonna seem wierd. Don't shoot the messenger.
An old cowboy around here uses used motor oil to treat wounds on horses. He swears by the stuff. He irrigates the wound with the stuff. Swears it reduces/eliminates the proudflesh you get on the more sever wounds.
I've never tried it, always sounded a little too strange for me, but all his horses are in good condition, and pleasant natured.
I've used it in horse stalls as an anti chew. Works well, cost's nothing and the horses don't like the taste. Does soak into the wood eventually and leaves a nice patina. I also paint it on the flooring of my landscape trailer.