Things like this make me glad I don't have a diesel. I don't spend $30 for oil at each change, let alone having the potential to spend 30 more for each change to use synthetic. I'm not sure what your 4000 miles equates to in time between changes, but it seems to me you're changing he oil a little too often. I think most vehicles can go between 7000 and 7500 or even longer. That's when synthetics shine anyway.