Scottoiler users....

[stevierst]

Used engine oil contains a few nasty things, unfortunately when mixed with water can chemicaly turn acidic, if you're a unicorn rider then it's not so much of a problem to use on the chain, however, not so helpful if you ride in the rain!

As I've said, I've been using it for years and never had a problem. I'm a 365 biker, and 30k/year. Never had a rotten chain, albeit a very grubby back wheel and swingarm!
Old engine oil has past it's usefulness in an engine, getting compressed, and heated, and cooled, but it's still slippery stuff when it comes out of the sump which is all a chain needs really. It's also pretty well filtered already, but it can't hurt to put it through a coffee filter. Each to their own I suppose, bit I'm too tight to pay for Scottoil that's too thin for what I need all year round, and in all weathers.
Gotta admit that I've got fresh engine oil in my oiler atm (asda cheap crap), but only because I ditched my last mucky oil by mistake in my garage clear out.