I use and have read on other threads that Castrol makes the BMW branded Synthetic motor oil. Many BMW enthusiasts are opting for the Castrol Edge with SPT 0W30 European formula. It will say "Imported from Germany" on the back. It apparently has better high heat anti-shearing properties and been independently tested to be better than the BMW LL01 motor oil. You can buy it on sale at AutoZone by the quart on occassion. My 545 take 8.5 quarts of oil so it isn't a cheap oil change job at all. I used to use exclusively Mobil 1 0w30 on my e39 528i. But since moving to the bigger engine and heat being an issue, I wanted to make sure I was putting in the best Synthetic I could get. I live in Tampa, FL area so it is hotter climate for most of the year.
As long as you are using a quality oil and changing frequently 5K to 7500 miles, your engine will love you back. 15K BMW intervals are just too hard on the gaskets and other seals. If you stay more frequent with the changes then you'll notice fewer maint problems over the long run.