I'm older than most people on this site, and thus, have probably owned more cars than most. I have a car lift in my garage and like to do as much maintenance and repair work as I can. I have sold and traded many cars...often simple sports cars and some pretty fancy Mercedes. I keep personal records of all work done on the cars using check sheets. I've never once had anyone question that the work was actually performed. Which means I do my own oil changes as much as I can. I also do changes that the factories no longer mention...change brake fluid, change coolant, etc.. Normally when a buyer of my car sees the extensive maintenance they don't question whether or not a dealer stamped a book.
All that being said, when I do use synthetic oil I normally opt for buying Mobile 1 at either Walmart or Costco. I've used synthetic on German cars, but not sure that they even need it. I've had cars run to 195,000 miles and with no engine problems whatsoever by just using a decent brand of oil and filter from any store. $200 and $300 for oil changes and a few checked items is totally idiotic in my thinking.
I suspect that BMW and Mercedes push synthetic oils becuase they eek out an extra 1-2 miles per gallon. Any thoughts on this theory?