Biodiesel is just like normal petrodiesel, it does not go bad per say, but it does grow allgee(green plant), why it looks like fudge is unknown to me. Any of the biodiesel I have ever made has a yellowish tint to it, even starting with some nasty looking brown waste vegetable oil. I probably would not run the fudge looking stuff. Petrodiesel and biodiesel are completely interchangeable, mixable at any concentrations. Your gelling(freezeing point) is higher with biodiesel than with petrodiesel. I ran some of this new ULSD stuff and froze up last winter, a first for me and when i took the filters off to replace them, it looked like a black slime, havent had any problem this year and we have been down to -25(without windchill) over nightand highs in the single digits above 0 for a few days straight. Good luck, Hope this helps some.