I bought an approved used BMW X5 that came with directional tyre treads at the front but asymmetric at the rear, all Dunlops. The BMW owner's handbook states 'incorrect tyre combinations will interfere with proper functioning of safety systems such as ABS and DSC. To maintain good vehicle handling always fit tyres of same brand and tread pattern to all wheels.' The BMW dealer replaced the rears as I found they had cuts down to the cords, but the tread patterns still don't match as the wider rear tyre size is only available as asymmetric, so I've still got a mix of tread pattern as well depths of 8mm at the rear but 3mm at the front. I think they should replace the fronts too but the dealer says as long they meet minimum required tread depth they conform to BMW standards. What do you think? Is this arrangement safe? Could it damage the AWD differential?