Take a look here.
Originally Posted by e92zero
I sometimes wonder if the BMW engine was really designed with a thinner grade oil from the start with the tight tolerance that BMRLVR pointed out. Then another BMW department decided to market and use the 10-60w for the prolonged 15k oil change interval. Would really like to know about the use of thinner oil, especially for some of us that change the oil at 5k or 7.5k.
I've run a mixture of 5W-30 Redline mixed 50/50 with Redline 40W race oil for the last two runs in my car. The current fill is 10W-40 mixed with the 40W. I want to get in some track time and see how the used oil analysis (UOL) comes back.