There should be a recall on that freaking engine!!!! Everyone that has a 4.8 is experiancing an exhaust smoke issue to some degree. Unlike the 4.4 where its usually just a crankcase vent valve, the 4.8 is the guides and stem seals. Replacing the stem seals is just a temporary fix if anything. The guides wear which creating play between the stem and the guide causing side to side play or a rocking motion. The seals, no matter how tight will leak from this rocking motion. A new pair of heads is the way to go. New heads with all nessesary parts and labor will run over $10K from BMW. Average cost for valve setm seals from BMW is over $3500 and they will not gaurentee valve seal replacement will cure the smoke. In other words you will be throwing your money away. How can a $70k plus vehicle have this on going problem and no recourse? I really think you will see a class action suit against BMW in the near future. Just my 2 cents.