According to German website Auto Bild, the engine would produce 150HP (112kW) and achieve a fuel figure as low as 3.9 l/100km.
Three different hybrid options will also reportedly be available, with an electric-only range of 50km targeted.
The six-cylinder options are all set to receive more power along with a 10 percent improvement in fuel consumption.
The M5 is expected to retain its current 4.4 litre V8 engine but with more power and torque, as BMW reportedly seeks to break the 600HP (447kW) barrier.
BMW may also be planning an auxiliary electrical system to power the growing number of electronic in-car features, with an output of 48 volts.
Recent reports have also suggested that the next M5 could offer optional all-wheel-drive for the first time, however an interview earlier this year suggests BMW M's Friedrich Nitschke may still be in two minds on the idea.
The all-new model is due to hit the European market from July 2016 in sedan form, with the Touring model (wagon) following in November 2016.
The updated-for-2013 BMW 5 Series arrived in Australian showrooms recently, priced from $79,900 plus on-roads.