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Look, load had nothing to do with the weight of the oil, weight is only a measure of the stickiness of the oil, that's why the high revving engines needs the extra sticky oil in order to stay on the metal parts, lighter oil will get fling off.
Simple experiment for you, grab a stick, dip it in water, than start swinging, does the water stay on? Do the same thing but dip it in honey, does it stay on?
Modern synthetic oil molecules do not break down no matter the load, unlike regular mineral oil, all synthetic oil has the same base unbreakable base molecules, the difference being the additives that's added that separate brands and grade. The additives are mostly detergent that keeps the engine clean.
BMW has specified the minimum API grade fro engine oil, Castrol 10w-60 and the Castrol 5w-30 that the X5/6M engine needs already met and exceed that grade, there is no added benefit of the additional 'additives' in the 10w-60, anyway most of the additional additives in the 10w-60 over the 5w-30 is for extending the viscosity range of the oil, giving it a 50 weight range.
Again, BMW spec the Castrol 10w-60 for the V10, V8, I6 engine only because of the high revving nature of those engines, the extra stickiness is NEEDED. It's has nothing to do with the so called 'extra sophisticated' TWS 10w-60, it's just a myth within the TWS fan club that the TWS is 'better'. BMW could have spec Shell Helix 10w-60 for those engines as well, the Shell is equal if not better than the Castrol, but Castrol was spec-ed only because they had a contract with BMW, nothing more.
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