I am wondering if it will not be a problem to use the same size tires but different brands on BMW X3. I got a pair of Pirelli Scorpion Zero 235/50/18 in front. I also got 235/50/18 BF Goodrich g-Force KDWS High Performance Tires which I'd like to install on rear wheels. Discount tires said that I cannot do that because it is AWD and tires have to be the same. Is any idea on why it might be a problem? Any advice is highly appreciated. Thanks.