I too believe the valve stem seals are not the issue on these engines. The positive crankcase ventilation is the problem. The crankcase vents to the intake manifold, both valve covers have rubber hoses going to the intake manifold, pull these hoses and look both inside the hose and into the manifold.
A small pen light will expose oil if it is there. On my son's car 2005 4.4L X5 145,000 miles both hoses showed oil, lots of oil. The manifold showed lots of oil. The spark plugs looked fine, leaking stem seals usually carbon up a spark plug.
The solution was to replace valve cover gaskets, the orings on the vanos and replace the diaphragms on each valve cover that control crankcase ventilation. The vacuum pump was also removed and new gasket installed.
This not only eliminated the oil leaks but the car runs and shifts like new again. Huge difference. My wife has a 2008 x5 4.8L and we already had to replace the vanos seals at 80,000 miles, it looks like one valve cover is starting to leak also.
When any one of these seals leaks it allows the crankcase ventilation to pull air into the intake manifold, this air movement will pull oil out of the valve covers into the manifold causing your oil smoke.
If only one hose has oil in it your problem is on that bank of cylinders, if both hoses have oil in them you need to do both sides. My sons car was pulling more oil from the pass side but the drivers side was leaking also, just not as bad.
The manifold should be removed and cleaned if it has oil in it. This fix is much less invasive than a valve stem seal replacement. All parts mentioned would be replaced if doing valve stem seals so people that do replace valve stem seals get good results.
If you have one or two carbon fouled spark plugs when your pull the plugs you need to look at the valve stem seals, if all your plugs look normal your problem is somewhere else.