When I picked up my A3 2.0TDI there was a bottle of Shell Helix Ultra in the boot - 0W-30, VW 504 00/507 00. I've always used Castrol Edge 5W-30 in my Golf GTDs - similar VW spec. Just wondering if it would be a bad idea to mix the two when topping up? I was surprised as I'd assumed Audi preferred Castrol.