Need to look at why the BMW synthetic was good but you needed a 40 series for the mobile. Most 30 weight oils do not have the HTHS (high temperature high shear) rating necessary while 40 and 50 weight oils typically do. That is why German Castol 5-30 is good but typical Castrol Synthetic 5-30 is not. Most of the "European Car" oils that you see are due to these requirements.
This is just a small sampling of this subject. Much more info out there. Head over to BITOG for more info.

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2003 X5 4.6is Estoril Blue
2005 Infiniti G35X
1972 Datsun 240Z
1991 Isuzu Impulse XS Sold
1972 Fiat 850 Spyder Donated
1972 Datsun 240Z Sold
1972 Datsun 510 Sold