The issue is not really the engine, but the fuel system and especially the pump and injectors.
Oil added to the fuel can be relatively thick and not burn efficiently or have problems getting through the system - hence slight loss of economy. There can also be some carbon build up, dislodging of debris in the system and in [rare] cases the oil can degrade rubber seals
Bio-diesel will be thinner and perhaps have some other additives to aid combustion - it is a true manufactured fuel to known standards.
Technically, bio-diesel is "designed" so that it works the same as normal diesel. Where as, using an ad-hoc alternative fuel mixture will work, but as the engine system is not really designed for it, the longer term effects are an unknown
Oil added to the fuel can be relatively thick and not burn efficiently or have problems getting through the system - hence slight loss of economy. There can also be some carbon build up, dislodging of debris in the system and in [rare] cases the oil can degrade rubber seals
Bio-diesel will be thinner and perhaps have some other additives to aid combustion - it is a true manufactured fuel to known standards.
Technically, bio-diesel is "designed" so that it works the same as normal diesel. Where as, using an ad-hoc alternative fuel mixture will work, but as the engine system is not really designed for it, the longer term effects are an unknown