If the mysterious video BMW released last week showing a mysterious M-modified X6-based vehicle does, in fact, show a diesel-powered X6, then it stands to reason we should be hearing more details about that new oil-burning M powertrain anytime now. And, would you look at that! Here’s a fresh, reputable-sounding rumor right here.

According to BMWBlog’s sources, the vehicle reportedly called the X6 M50d will produce 381 horsepower and 545 lb-ft of torque from a tri-turbo diesel-powered inline-six—down 174 horsepower but up 45 lb-ft over the gas-powered X6 M. If the rumor is true, though, the X6 M50d will stampede from 0-60 in 4.9 seconds, only four-tenths behind BMW’s official figure for the X6 M. With production supposedly slated for April 2012, we’re guessing we’ll see the official X6 M50d announcement slide into our mailbox any day now.

BMWBlog also reported that the same engine will indeed be slotting into a 5 Series (purportedly dubbed M550d xDrive) in March 2012, where it’ll push the sedan from 0-60 in only 4.7 seconds, all while returning around 38 miles per gallon on the European driving cycle. Sadly, that car is reportedly not scheduled to come to America’s shores; that said, if the X6 M50d proves a hit here, maybe we can convince them to give us a crack at the torquiest 5 Series ever. [via BMWBlog]