I may be wrong but I believe when you fill up with “biodiesel”, you are getting between 85% and 95% petroleum.
It depends on the blend. If you get B100 you are getting straight non-petroleum based oil. A lessor number means you are getting a blend of petroleum-based oil and a non-petroleum based oil. B80 is pretty much standard here.
Most of what I have seen for sale is B20, or 20% biodiesel, 80% petroleum diesel. There is also B100 (100% biodiesel) for sale some places, but I’ve never seen it. I don’t know if they change the blend in the winter time like they do with ethanol. E85 is 85% ethanol, except that in the winter time they make a “winter blend” that is a good bit less than 85% ethanol because really high concentrations of ethanol in in your fuel makes it hard for you to start your engine in the winter time. Biodiesel will gel when it gets too cold.