IO540
I say again IMO there is no problem leaving the mixture full rich in the average club trainer/tourer for climbs to 6000/7000 ft. In fact it is the safest way to fly these minimally instrumented aircraft by low time new pilots like the original poster in this thread
Leaning so as to achieve the serrvice ceiling, management of the mixture control at 20,000 ft , constant EGT climbs etc are important concepts but they are advanced flying techniques so therefore I would suggest that you start a new thread to pursue this topic as I would suggest that they are concepts currently beyond the scope of experience of the original poster.