I only use "real" synthetic oils made from synthetic base stock. Most "synthetic" oils sold in the U.S. market are actually not from synthetic sources. Overseas, as in Germany, these oils are not sold as "synthetic", but "heavy duty" to the best of my knowledge. Mobil 1 used to use to be a "group 4" (real synthetic), but I'm not sure it still is. I believe the Mobil 1 ESP is, that's why it's ok'ed for 15k change intervals. That's what I use on my truck. Other "real synthetics" are Redline, Total, Elf, Motul, Pentosin, and Lubro Moly synthetics. A lot of these can be hard to find and originate overseas, but are approved for extended drain intervals (over 15k miles). They are worth the extra cost in my opinion. In my BMW M Roadster, I use the only approved oil, Castrol TWS 10 W 60. Check out the website, Bob the Oil Guy, you will learn a lot.