C172 is modern, but it's flight manual is not?????
You should have the correct flight manual for the particular aircraft you fly. If you don't, then don't fly it. If you do then fly it within the limits that are defined in the flight manual including any supplements that it may have.
And you should use a rich mixture if you are going to use high power, but you may not be able to get high power in a 172 engine at 6000 feet. It will more likely run rough due to being too rich. If it does, then you can lean it intil it runs smoothly. Check your flight manual and/or the engine makers manual. Even if they are old, as long as they apply to the particular machine you have.