I know I am going to get some "slack" on this post but I DO run a 50/50 mix of Stihl and good clean motor oil and I have been using it for 3 years now and can HONESTLY say that I cannot see a damn bit of difference. Like one of the previous guys posted.... chains and bars are relatively cheap and you can easily spend that equivalent in expensive new Stihl (or other brand) oil. I have yet to wear out a bar and yet have saved $$$ in using the 50/50.
P.S. the old motor oil is strained through paint cone filters so it is as clean as it can get. I change my oil at 2000 miles so it is not all that dirty.
Tree Raptor
Used motor oil is classified as toxic waste, and it may well be illegal to spray it out in the woods. Especially in sensitive watersheds. It destroys underground aquifers as sources of drinking water. Bar oil is not that darned expensive.