This has been brought up before. Basiucally as long as the oil you are using is an approved oil on the bmw list... the most likely cause is water condensation in the oil.
Do you do lots of short trips? Is your engine oil cap all milky sludge color? Try a long trip to burn off the water in your engine oil.
Another thing that can cause this problem is your SW version. Early SW versions had a known bug - bmw's fix was to update the engine SW.
Lastly... some sensors do infact go bad. Have you checked your car for any codes?
IF your car(s) are still under warranty - just go visit a dealer.
here are some handy pdf's from bmw...