I did a little test back in June. I made a 50/50 mix of waste veggie oil and diesel and put it in a mason jar. I did this test because EVERYONE says that they will separate. I left the mixture on the table in the basement undisturbed. I went downstairs the next day and there was no separation. I went down and looked at it several days after that and there still was no separation. To make a long story short, I just put that mixture in my tank last week and, after more than 2 months, there was no visible separation of the oil and diesel. I also tested this same batch in the refrigerator and it did not gel.
I have been running a 25% blend in my truck all summer with no issues.