The stated oil weight doesn't mean a whole lot, as there is lots of room for variation within each SAE grade. I would look at the 40 degree and 100 degree viscosities to better compare oil. However, for BMWs, the biggest thing to look for is LL-01 approval. That means far more than the stated viscosity. Having said that, the Castrol is notably thicker than the Special Tec, which should help with clatter.

It's too bad Mobil 1 0W-40 lost LL-01 approval.

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2007 E70 4.8i