Originally Posted By: bmwtechguy
15W-40 is just one notch up from the recommeded 10w30. How could that be "way too thick"? Auto-Rx does not cause leaks. However, an engine that was very neglected for oil changes or used inferior quality oil over the years may expose a leak as the seals are cleaned and then seal up later in the rinse cycle or during a second treatment. Look in the oil fill hole at the top of the cylinder head and see how much crud or deposits you can see with a flashlight (torch?) This will give you an idea of how things are from previous owner's maintenance.
Because every N/A 3800 i've owned spec'd 5w30 as the primary grade as most GM cars do
the only 3800 of mine that did not was my '96 SC Bonneville,and that spec'd 10w30.