BMW (and other manufacturers) evidently do have Castrol add a "bit extra" to their OEM oils. For instance, my 335d uses a BMW LL-04 oil that is based on Castrol SLX. However, it's interesting to compare oils: VW specs Castrol SLX LL03, which is rated ACEA A3/B3 C3. BMW's LL04 SLX for the 335d also has the same ACEA rating (severe service low-HTHS, extended drain, low ash). It's come to the point that the SAE ratings are useless for modern BMWs.