I myself believe in shorter oil intervals on these engines. The average person has short commutes and my not run the engine a high RPMs which can account for sludge and not completely flushing the oiling journals. This is a common occurrence on newer BMW's My 540 had visible sludge on 15k intervals but my 545 has no visible sludge and I have been doing 7500 changes. I would be inclined to do 7500 intervals because of the Turbos. They do a very good job of heating the oil up. 250 degree oil is not to difficult with a little spirited driving and from my past motor experience, is not high oil temps are not good.