i use the stuff from the farm store. in the fall they will carry the winter grade as well as the standard grade. i also scored a few gallons of pouland brand last year when the wally world was moving to a new location.
in a pinch i have been known to dilute my bar oil with karo, motor oil or a splash of fuel mix. i wouldn't ever use used motor oil for the reasons whitespider mentioned.
the stealership claims there is quite a bit of difference in bar oils and cost because of the additives which are employed to maintain proper tackiness, viscosity and the actual lubricating properties involved. the stuff has to lube 1st and formost and it has to cling to the bar/chain while it also flows between the links and straps. that seems to be a tall order we're asking from our bar oil. that's the reason i try to stay ahead of the game and keep both standard and winter weight on hand for my saws.
no different from engine oil. castrol synthetic is a far better lube for your truck than say tech 2000/generic engine oil. one is a better lube than the other because of the quality of the product.