2008 Mini Cooper Oil Problem

We don't memorize the capacity of every engine because our little minds are filled with important stuff. All we do is pour in about the right amount, run the engine just long enough to fill the new oil filter, then check the level and add as necessary to bring it between the "minimum" and "maximum" marks.

Overfilling by a quart or less is not going to cause a serious problem, and certainly not a leak. The oiling system is sealed for emissions purposes, so there's no place for over-filled oil to run out. It sounds like you just have a typical leak that needs to be diagnosed and corrected. If it's a fast leak, it will be obvious when you look for it with the engine running. For relatively slow leaks, you can wash the area, then add a small bottle of dark purple dye to the oil. Search a day or two later with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. Auto parts stores have the dye, and those that rent or borrow tools should have the black light.