We are on our tenth n54 that is in need of valve stem seal replacment this year. It seems like it's becoming the norm for higher mileage n54s. 70k miles and up. We have been getting quite a few smoking n54s, not all have just been valve stem seals, but upon removal of the turbos we have seen about 80% of the cars needing them that come through our shop. Heres is what a leaking valve stem seal would like look wit has the turbos removed Special tool needed to release tension of the valve springs so the cam can be removed, without this you run a risk of cracking or damage. Shop air is forced into the cylinders to keep the valves in place sof once the keepers are removed it does not fall into the cylinder. Our special tool needed to be able to replace them in the vehicle, the conventional way is removing the whole head and doing it on a bench or such.