Fix thermal/conductivity with different atoms

Dear Lammps users:
I want to use "Fix thermal/conductivity" to compute the thermal conductivity of system having different atoms.
The manual says "This effectively swaps their kinetic energies, assuming their masses are the same".
So I don't know whether this command is suitable for different mass.   
The command I used is "fix  all  all thermal/conductivity 10 z 20
                                   fix  C  carbon thermal/conductivity 10 z 20
                                   fix  O  oxygen thermal/conductivity 10 z 20"
However the swap energy of all is not equal to the swap energy of C plus the swap energy of O.

Further down, the doc page says:

Swaps conserve both momentum and kinetic energy, even if the masses of
the swapped atoms are not equal. Thus you should not need to
thermostat the system. If you do use a thermostat, you may want to
apply it only to the non-swapped dimensions (other than {vdim}).

So the line you quoted is out-of-date. I will edit the doc page.