There has been a leaked document from BMW that suggests that the German automaker are considering a M5 diesel. M550dX is what badge it will wear and we all know what the M stands for as well as the d but X means all-wheel drive. The engine could be the new 3.0 litre triple turbocharged motor that produces up to 400hp. That will be some serious performance in a diesel! In all honesty it might just be a 550d all wheel drive with the M sport package but lets be optimistic here.