My 2005 X5 3.0 (51K miles) has produced a sizable cloud of blue smoke upon cold start on three separate occasions, spaced several weeks apart. The clould is large enough to be seen in the mirrors but only last for a few seconds. The car has had 8K mile oil changes with BMW synthetic oil and has never required additional oil between changes. There is no visible exhaust smoke except for these few incidences and this appears to be an intermittent problem. Otherwise, the engine runs like new. The dealer wants me to bring it in for diagnosis at the $120/hour shop rate. Before doing so, can anyone speculate as to what's causing the problem?