The POH of our Archer specifically states that the 'normally' aspirated O360 should be operated full rich at anything below 5,000', so that's what we teach the students.
Nope...it specifically states (!) this...
The mixture should be leaned during cruising operation above 5000 ft.
altitude and at pilot’s discretion at lower altitudes when 75% power or less is being used.
Reference
http://www.aceflyingclub.com/manuals...28-181-POH.pdf
Para 4.14