My question about the use of motor oil as a wood preservative has been answered by the honorable Stephen A. Shepherd, who is a walking encyclopedia of historical woodworking practices. He is unable to post comments on my blog because, apparently, Google requires that comments be posted by people who have accepted the twenty-first century. :-) In an email, though, Stephen tells me: "I have heard of using motor oil, it is cheaper than watco or orange oil and about the same thing. I have also heard that it causes the wood to deteriorate, as I have experienced looking at old benches in metal working shops. That is why I don't use mineral oil, it is a laxative and I am a regular guy anyway. I would stay away from motor oil for wood, petroleum distillates and wood don't go good together, both aesthetically and chemically."
Thank you, Stephen. For more of his wisdom, visit his Full Chisel Blog.