my reading up on a while back, was if you just bought certain types of vegetable oil and mixed it with diesel, depending on whether it was winter or summer was all you had to do,
Where I buy my bio diesel, and these people supply bus and coach companies, ferry operators, etc, the bloke there just said 'DON'T'. I seem to remember the problem is in the mixing although some lighter oils can be used for a while.
If the engine is hot, then almost any oil can be used untreated. It's when the engine is cold that there is a problem. There is also a further problem of the wax going solid in the oil in very cold temperatures and clogging up the fuel filter in minutes.