Oil costs - budu
Rather than re-use fish fryers' oil, take a leaf from the book of a garage proprietor I knew. He saved oil from customer car servicing, filtered it and used nothing else for his three family cars, one business car and a breakdown truck. He reckoned it was o.k.
Oil costs - andymc {P}
No matter how well he filtered it, it's an option to avoid. If you think old diesel engines are smelly this stuff would be ten times worse - vastly more toxic and polluting than ordinary derv. Air quality in towns would be like London in the smog days. Definite MOT emissions failure as well.
Oil costs - madf
Air quality in London - or parts of it - fails all known standards at present.

Frankly I would never use recycled cooking /other oil. What you save on fuel costs is likely to be lost in gummed up injectors, coked up engines and fouled catalysts - in the long run.

And for older diesels which have fuel pumps that rely on diesel for lubrication , I suggest they may be affected as well..