I would think that the free fryer oil from restaurant bit is a transient remnant of when they had to pay to get rid of it and there was no demand. As soon as the demand will build up and people will start shoving each other at McDonalds to get the oil, they would be stupid to give it away for free any longer. I would not plan on free or dirt cheap waste oil for very long and I would certainly not count on that in estimating the cost effectiveness of a diesel swap.