I recently finished replacing the seals on my '04 545i. I used AGA's special tool and the task, along with coolant transfer pipe replacement, took me about 32 hours of labor. Once I got to valve covers off, each valve stem seal took an average of 25 minutes to replace. There are 32 of them! I made the repair at approximately 125k miles, but seals were leaking oil for some time prior which I believe contributed to my clogged secondary air ports. I was able to clear on side, but the other side will require removing exhaust manifold and mechanically clearing clogged ports with AGA tool for same purpose. Saving that task for another day.....

Also, my 08 X5 4.8i has 81,000 miles and the valve stem seals are fine for now. Fingers crossed.