I think it's a myth! Only thing I heard makes sense is type of oil they use initially is not the same viscosity as the one bmw puts into our cars.
The reason is it's special oil to get rid of metal shavings or residues in the engine. Not sure how true is that! But if it is , it has some sense, why we feel the difference . I personally didn't feel anything , but anyways I didn't push it before break in so I won't know
