At a minumin I would want to raise its temperature to say 450 degrees to get rid of any condensation and anything else that would boil off. A long settling period afterwords might separate/settle the metal products or any element that is heavier than the oil. Then filter it. Ph strip tests might be a good ideal to check that it is not too acidic as well. Just removing from an engine and dumping it into a cars tank might turn out to be false economy. But I really know nothing about alternative fuels. I just try to keep an open mind.