This reminds me of the hot debates about which trailer axle grease is best. Some swear by red and tacky, others only use this $$$ brand or that, others use Walmart's SuperTech, and all have good success with each.
"Grease is cheaper than steel." It seems to me that as long as the bar stays oily, things will wear less than if it's running dry, regardless of what particular oil you are using.
Used motor oil is thinner, and auto lubes more liberally, so it seems to auto-correct for it's lesser lubricating properties. I've never had any problems with it, but I find it slightly annoying that I have to fill up more with it. I'm really happy with using used gear oil, and now I'm going to start using hydraulic fluid, too.
As far as used engine oil, I get way more than I can ever run through a saw. We run multiple vehicles, tractors, ATV's, splitters, pressure washers, tractors, mowers and more. I've been giving most of my used oil to my local auto mechanic, but I'm considering buying a waste oil furnace like he has. They keep their shop nice and toasty, and it costs them nothing. I just need to find one cheap enough.