It is possible that the PT6A-68C has a modified Oil Scavenge system to allow the aircraft to perform aerobatics or inverted flight, but as the -68 Series covers both the Texan II and Pilatus PC-21, I suspect that the modified oil scavenge system is standard to the -68 Series.
The C is the most powerful of the -68 Series and having built the -68 and -68B, it would be logical to name the next variant the -68C.