In the example given, I think that "12 o'clock" and "opposite direction" are enough of a clue that a turn in either direction would be a better option than remaing on the current heading.
Unfortunately, that's not the case. You're doing 150 kt. An aircraft reported in your 12 o'clock, "opposite direction", at 5 miles may be passing safely down your right side by a mile. A right turn of 30 degrees puts you perfectly on to a collision course with him if he's doing the same speed as you. More than 30 degrees then? What if he's just a little faster than you?
If the controller simply said "turn left to increase separation" or even described the encounter as "left-to-right" which, in terms of relative bearing, it is, the action to take is obvious -- you turn to the left.