My cousin who is a mechanic has helped on occasion with the x5, and always recommends me with mobile 1, i never asked why even though i see that bmw recommends Castrol. Even though Bmw recommends Castrol u dont have to always believe it, they may have been paid to do that. Iv seen a previous post of a member seeing BMW recommended BP for gas because bmw considers them to be top tier while it really isnt. I am no sure what the difference between the two synthetics are but i trust my cousin and haven't seen any gunk left on the dip stick. Just my 2 cents

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2005 Black x5 3.0i