Here's my 2 cents. I have been using synthetic oil over twenty yrs, racing motocross and auto cross. Synthetic oil is the best choice. Many will argue about Royal Purple and all the other premium brands are so much better. Keep this in mind. I own my X5, with the N55 engine. Free BMW 15000k oil changes don't work for me because I don't want to wait that long. My oil is changed every 7500 miles with Castrol euro oil and Mahle filter. Living in South Florida, engine temps are quite high, taking its toll on breaking down oil and coolant.
I was also told by a certified BMW mechanic that long intervals between oil changes cause VANOS issues, yet BMW won't talk about it. For that reason I won't wait every 15000 miles. If leasing, I wouldn't worry about it. Just remember to keep those awful riding runflast within BMW specs on lease return.
Again this is my opinion, by information provided to me by professionals.
Lastly I plan on keeping my X5 till it's at least 10 years old. The extra $100 per year is worth the cost. Oil does break down and 7500 miles is my max on all my cars, regardless of what is said. Some people will want to argue over this.