I have 88,000+ miles on my 2001 4.4i. While under warrenty I used Gastrol Synthetic 5W-50. Since warranty expired I have used Mobil 1 15W-50 (I live in Houston; would be too viscous/thick for extremely cold Northern climates).

I've pulled a 31 foot 8,300 pound GVWR travel trailer over 30,000 miles with my X5 and the higher viscosity engine oil is best for a highly loaded engine, but is equally beneficial for the longevity of any engine regardless of load (all you'll sacrifice is a fraction of a mile per gallon of gas due to greater fluid resistance).

__________________
A good pessimist is never dissapointed!