Protecting barn with motor oil?

We are getting a barn built next week. DH is trying to figure out what to paint/stain the outside of the barn with to preserve the wood. Most people around here paint their barns with used motor oil and have done it that way forever. The idea sounds horrible to me, but I've read that it isn't any worse than using things like "Thompson's water seal" and other oil based stains. I've also seen it suggested as a way to keep horses from chewing wood (they won't be stalled in the barn - its really more of a run in shed and they'll be out in the pasture most of the time, so I don't think it is too likely that they'll chew on it anyway.).

So, has anyone ever done this? Any problems?

And finally, husband wants to know if synthetic motor oil will work just as well, since we have a bunch of it sitting around that needs to be recycled anyway.

I would use the burnt synthetic that you have on hand and hit your neighbors up - We have done oil changes for our neighbors for $10, they provide new oil and filter and we keep the used oil. Works out so everyone is happy.