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A good synthetic oil, proper heat management, adequate supply, and many vehicles could go in excess of 20K miles without compromising the protection. There is only one way to know for certain whether the oil is losing its effectivity - take a sample and test it. Cost is about $25, but then you have to get the sample first! Considering BMW warrants the vehicle if you change it when their sensor says to, I feel you should be fine waiting for that to do it. Only if I lived on say a dusty road, or maybe in the desert where you got a lot of blowing dust and sand would I elect to change it earlier.
I'm waiting for BMWUSA to bring the 530d GT in, but until that happens am driving my M35x. This car specifies 3750mile oil changes, and with modern oils, I thought that was totally obscene. I've been running synthetic oil in it, and having it analyzed (at first about very 4K) to see how it faired. Well, on my car, I can get about 10-11K before it needs to be changed. This is with a high-reving vehicle (over 3Krpm at 70mph) engine with a 5-qt sump. the turbo engined BMW is geared to turn much lower at cruise, and has more oil in it. I would expect no problems running the oil much longer. Another side benefit from checking the oil is you get a heads up if something is wearing out. In my last analysis, it showed a small amount of silicon and phospherous, indicating a slow coolant contamination (no water indicated in the oil). Not enough to notice the tell-tale sludge from a bad leak, or to really notice a loss in the reservoir, but a heads up (pun intended!) that there was a problem developing. Tried some new Bars Leaks treatment, and will see if that did anything the next time it is analyzed. This is a new product from them, designed to lubricate the water pump, and aid in maintaining the seals. They claim some manufacturers install it in new cars at the factory. I won't know for awhile if it did anything, because the only idication I did have was from the oil analysis.
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