It used to be in the old days(back when we used to walk to school uphill both ways with no shoes, and spikes driven through our feet for traction), that factories would put 'break in' oil(ie ****) to help rings seat, and all the other metal on metal contact break in, and people would switch out to regular oil rather quickly. Now days with modern design software, metallurgy and the break in they do at the factory, there's not much break-in compared to the old days. Matter of fact, a LOT of cars come stock with fully synthetic oil from the factory, which is also so good now days that wear(or 'break in' for the purposes of this thread) is almost non-existent with it. And whoever said mileage doesn't increase with synthetic oil is absolutely WRONG. If you compare apples to apples, synthetic oil creates less drag on the rotational idgits inside our motors, thus makes them more efficient, ie more powerful and with better mileage).
Back when I was a young sprout, I used to road race motorcycles. Everybody and their brother uses synthetic in racing engines for that very reason. Unless you're doing long endurance races, it's definitely NOT for reliability reasons. We used to run ZERO-WEIGHT Maxima synthetic in our motors, and in a 160hp 1000cc 'Supertock' engine, would gain several hp over dino oil. Again, common knowledge in racing circles. We'd occasionally also do endurance races(4-6hours), and would also run the zero-weight in those motors as well, as mileage was the name of the game to limit pit stops, and over the course of 400-600 miles, you'd gain LAPS of 'extra' fuel by using it. It wasn't so much that it gave an advantage as it was a disadvantage if you DIDN'T use it.
Anyway, back to my original point, VW actually requires you to bring in your diesel vehicle around 900-1k miles for its first oil change, but they were the only ones that have/had such a rule that I've ever dealt with. Traditionally speaking, changing your oil(even your first batch) at 1000 miles is simply a waste of good oil. It gives some people peace of mind however, and as we all know, that can be priceless.
As far as Shell Rotella is concerned(someone asked about it), Shell manufacturers this oil for semis, construction and farm equipment('Heavy-Duty' diesel engines). A lot of guys with 3/4 and 1-ton trucks use it. It's great oil, but as I said, it's made for large, 'heavy-duty' diesel motors.
Furthermore, IIRC Shell is coming out with a revised formula for the new(er) crop of HD diesel motors that have increasing higher mileage, and lower emissions requirements placed on them.
Because of this these motors run much hotter, which can break down internals quicker(they have more stress placed on them). Despite Shell saying that this new oil won't cause more drag, I wouldn't be a bit surprised if it did, as it's job is to survive in more extreme conditions.
Because of this, I personally won' t be running it in my relatively tiny 4cyl Duramax because it's a totally different motor, which has different requirements than a 16-liter Mack MP10.
Of course, YMMV
Oh, my take on synthetic blend is it's simply watered-down synthetic. Reminds me of guys that cut race gas with pump fuel. All you're doing is watering down the good stuff.