In conclusion, and based upon extensive research from many reputable organisations, the best advice is only to use REFINED base oils, preferably from reputable suppliers, and do not use plain supermarket vegetable oils, or partially refined (straight vegetable oils) or unrefined waste cooking oils in diesel engines.
Stick to refined oils from reputable large scale fuel producers (the big boys) or reputable small scale producers who will produce a product to the relevant standards, and have the quality testing procedures in place, and are willing to show their records of these quality procedures.