BMW Repair: Synthetic Oil, independent repair, 740il

Question
I have been running 5-30 BMW Full Synthetic oil in my 1998 740IL for some time now and have noticed a slight rattling noise at low rpm (idle). After changing the oil today the noise seems more noticalbe. The car only has 70K miles but should I be using a heavier oil (15-40)?

Answer
Hi Rob,
The problem is not in the oil but the real cause of the noise.
Considering your low mileage, there is no reason that you should use high viscosity oils--they would rob power, decrease mileage, etc. More importantly, it may mask the real cause of the problem which can be dangerous.
Bring the car to a dealer or independent repair shop that knows how to work on BMW's (not just any shop), and have the source of the noise diagnosed.
Pay special attention to the VANOS (variable valve control infront of each engine bank). They tend to be noisy at start up when idling.
Then, when you are sure that there is nothing really wrong with the engine, you may use 10-30 synthetic oil or semi-synthetic oil.