During the summer months, I actually run the "M" oil in my 3.0 X5. I believe it is 10W-60. Living in Texas as you do, the 100+ degree days result in noticeable consumption of the normal 5W-30, to the tune of about 1 quart every 1000 miles. With the 10W-60, I usually go 2000 miles on a quart and have a lot higher confidence level in engine protection while towing the boat. There's just a little slower engine cranking on a cool start, but no measurable drop in fuel mileage.

2002 X5 3.0 237,000 miles
2004 325i 106,000 miles