Buy a diesel and have the conversion so that it can use normal oil mix
There are a team of guys in the UK that have converted loads of vehicles, including a brand new Mercedes (all diesel) so that they can run on used oil that you can get for free from restaurants in those big barrels.
The government was a little miffed but have accepted it and stuck a tiny amount of tax on it... If more people did that, it would scare the bejesus out of the oil companies and trust me, their prices would drop...
btw, one of those guys drove to Portugal and back from the uk on that stuff and it works fine..