BMW planning M5 with diesel, all-wheel drive?

It wasn't that long ago that a BMW with all-wheel drive – or really, anything other than rear-wheel drive – would have seemed downright apocryphal. But these days the Bavarian automaker offers traction at all four corners on a growing variety of vehicles... including the X5 M and X6 M crossovers. But an M5? If you shudder at the thought, you may want to stop reading here, because word on the street is that BMW is not only planning to place an extra driveshaft in the M5, but also give it – gasp! – a diesel.The rumors are sparked by an apparently official document that includes a model designated M550dX. The first letter and number we all know and love. The 50 reportedly refers to a new triple-turbo diesel inline-six set to be unveiled at the upcoming Frankfurt Motor Show. The D, of course, refers to the diesel fuel it will burn, and the X for all-wheel drive.Although most M cars these days carry simpler names like M3 and M5, enthusiasts will note that one of the division's first products was the M535i of 1979, which came along long before the first M5 in 1984. Could BMW be planning on returning to this nomenclature for its first diesel, all-wheel-drive performance sedan? We'll have to wait and see.