Originally Posted by av2
Like someone mentioned, I would change the oil out earlier than BMW's suggested interval. The factory TWS motor oil is high a quality group 4/5 synthetic. Adding just a liter of mineral won't harm your engine but it will degrade the oil inside your engine quicker. The viscosity of the oil added was thinner at operating temp plus group 1 and 2 mineral oils won't last 10,000 miles. Mineral oils should be changed out within 3,000 miles to avoid sludge build ups.
Finally, some good advice. Thank you!