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One of the std OEM diff fills is Castrol SAF-XJ. The Noise reduction fill is Castrol SAF-XJ to which BMW has added extra slip agent. This fill is only given to customers who complain about diff noise. The question you have to ask yourself is why BMW don't make the OEM fill the Castrol with extra slip agent if there was no detrimental effect? (The cost of the extra slip agent is pennies.)
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