The 1st O2 sensor is used for Air/fuel metering and the 2nd O2 sensor is used for emissions. So yes the MIL will only help if that is your actual problem. If you don't have any issues with air/fuel ratios a MIL will probably work for you.
Melb you car was running very rich so that would more indicate a problem with the 1st sensor.
Of course we don't know for certain what is happening inside the
ECU and can only speculate.