Well don't forget to refine it as much as possible, the more times the better and also replace your fuel filter more often..
As for working, it will work absolutely fine, Diesel engines were orginally made to be run on peanut oil.
You could apporach some of the big fast food chains like Mac D or KFC. Tell them that they could publicise the giving away of the oil. They love nothing more than free publicity.
It is a great idea, and would be great if you could find a regular source of used cooking oil plus your exhaust would smell of fried chicken
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Just be sure not to dump it straight into fuel tank