Hi Mak
First Both Lycoming and Continental state you can lean their engines until they stop AS LONG AS THEY ARE ON 70% POWER OR LESS.
Given enough time I could find the service letter numbers,back in the late 70,s or early 80's.
The airframe manufacturers try to make it idiot proof by stating that on normally asperated engines you should not lean below 5000 feet - at this altitude the engine is only producing about 70% power !!!
Second:- as you pull out the mixture control the engine eventually runs rough, this is because the engine has a carb. the fuel distribution between cylinders is not even ,so one cylinder is generally running slightly leaner than the rest. So as you lean ,this cylinder stops working first - thus rough!!
The best you can do is then ease the mixture back in until the roughness has abated.
If you try leaning up fuel injected engines there is no roughness,
they just stop producing power because the fuel distribution is usually very good.
As an addon , the most economic is at full thottle altitude say 6000 feet (60% output) and lean .
Regards