I know some people who use synthetic motor oil to lubricate the moving parts on their AR's, so that part doesn't sound too strange to me. As mentioned before, motor oil doesn't have anything in it that will cut copper or lead fouling, and it will be marginal for removing carbon. My personal preference is to clean with a good solvent like Hoppe's #9 or KG 1. I prefer to lubricate with a quality CLP, but a little Mobil-1 will do the trick for that, too.