An interesting thread.
A few years ago I purchased some equipment from a UK company (Green Fuels ltd) which enabled me to refine the local restaurants’ used cooking oil to a useable diesel substitute.
The basic oil, if just filtered, will very quickly block fuel lines & filters due to the content of animal fats, water & other strange additives. This will happen very quickly in winter.
Cooking oil contains a thickener, glycerine, which needs to be removed too, otherwise the oil will be too gloopy to flow properly unless it is quite warm.
The process is quite simple though & the cost of the methanol, electricity & other bits meant a per litre cost of 15 centimes
I ran my Volvo V70 with it & also my military Reo M35 multifuel with it.
I was stopped once by the Gendarmes for a document check & although there was a hint of cooking in the air (my engine was still running) there was no issue with my fuel.
I am not sure of the french tax stance on personal biofuel but I can imagine the uproar if a newpaper headline read “Man punished for being environmentally friendly!”
I met other french motorists doing the same but none mentioned anything about it being illegal.