Environmental concerns aside, which I think are negligible in the grand scheme of things, I contend that there is no way used motor oil can protect your bar and chain as well as specified bar oil. I have been a bicycle mechanic all my life and owned and operated a shop for 15 years. Not chainsaws I realize but there are some parallels. On my workbench there would always be at least a half dozen different varieties of lubes. The same oil that was ideal for chains was less than ideal for cables, freehubs, etc. Many granolaheads frequented our shop and used all sorts of concoctions to lubricate their chains, including used motor oil and vegetable oil in their endeavors to save the planet and some cash. Vegetable oil is the worst, being a magnet for dirt and grime and would create such a mess as to be nearly impossible to clean. Used motor oil fared not much better. These people would always swear by their methods but I could see the damage being done. Prematurely worn chains, sprockets, and chainrings were the direct result of dirty and/or wrong lube. I can state with absolutely no doubt that the #1 cause of prematurely worn moving metal components (on a bicycle at least) is due to dirty and improper lubrication and the accelerated wear can be dramatic, 3 or 4 times more than the same part under ideal conditions. When I change the oil in my car it is black, stinky and just plain worn out due to heat and friction. I doesn't feel slick like new oil, that's why I'm changing it. Maybe none of this has anything to do with chainsaws, but I'm wagering it does. I know many have stated good results with used motor oil and I'm not disputing it, just saying that I think you would fare better using clean lube. I don't look down my nose at anyone and appreciate thinking outside the box, I am really a fan of directly recycling whenever feasible. To each his own, absolutely! Until I hear otherwise from some experienced saw mechanics, ones who have made their living at it for some time, I gotta believe that a clean lube designed to hold its place at high rpm's is going to save you money in wear and tear in the long run.