Ok I just talked to my local BMW shop service manager, and he said that even with the older E53, there is no need to replace all 4 tires at once, and that it is actually quite common to replace the fronts before the rears, especially for people with "spirited" driving styles. He said ALWAYS replace left and right at the same time though.

As for mixed brands, he said it is defintely better to use the same brand / model of tire, but if you do change, try and go for a tire in the same performance category and load rating.

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