I've thought of using synthetic oil and if I'm going to do so it's about that time (mileage wise on the engine). Oddly most diesel mechanics I know say to stick with the regular oil such as Rotella T 15W40. I do know one fellow that uses synthetic and swears by it so I'll have to take a closer look. "Thanks" for the reminder.
The sometimes higher mpg diesels give you is nice but this last trip I made to Indiana I never got more than 12.2 and that was in the last 150 miles. Most of the trip I averaged around 11.8, maybe because I was pushing the engine a bit. I guess even at 11.8 I should be happy but you know how it goes, we always want that extra couple of miles-per-gallon.