I've heard similar stories from people who have worked at refineries - especially fuel delivered to independent stations - in my own experience: I use to own a sprinter 5cyl diesel - cheap fuel from Husky it ran like shit and it was almost an instant change - I found Sunoco had the best diesel but they where few and far between but a very noticeable difference on fuel mileage and operation - good diesel made a noticeable difference especially in the winter