There are a number of pros and cons regarding the use of vegetable oil in diesel engines.
The most glaring pro is that Herr Rudolf Christian Karl Diesel originally designed and exhibited his first compression ignition engine to run on groundnut (peanut) oil! It was only the avarice of the petrochemical industry that convinced the public that they needed to buy a hastily modified waste product
of the crude oil cracking process to use in these engines. It was around this time that Herr Diesel ... mysteriously ... disappeared on an overnight Channel ferry
Perhaps the strongest con is that most modern 'common rail' engines - and many older ones using Lucas injection pumps - just don't like it and can fail drastically. In such cases it is best to stick to either properly converted bio-fuel or just a 50/50 mix of oil and proper
I've been running a 1996 Citroen Xantia 1.9 litre Turbo Diesel (Bosch injector pump) for over a year and many ,000's of miles on 85% WVO, filtered to sub 5 microns, with 15% unleaded added to reduce the viscosity and the cetane rating. Since I gave it to my niece I have supplied her with more oil and, so far, it's only had the one replacement fuel filter and even that should
have been replaced when I first bought it!
Ps. The car has now covered well over 205k miles and still returns 50mpg on the WVO/unleaded.
If I can't break it nobody can. Failures to date: Nil!