"Is leaning an engine to lean of peak a good habit? Can it have a negative effect on engine life?" - Gene B.
“Always follow the POH for your aircraft. Lean of peak operation may be prohibited with your aircraft.
If not prohibited, significant reductions in fuel flow can be achieved running lean of peak, but proper instrumentation such as a graphic engine monitor with an accurate fuel flow indicator must be installed to monitor conditions in all cylinders to avoid damage.
The technique used to adjust the mixture to the lean side of peak is key in avoiding engine damage. The mixture must pass through peak EGT operation to get to the lean side of peak. This must be done quickly to avoid EGT operating conditions where high cylinder head temperatures and subsequent engine damage might occur.
Also, some engines can’t run smoothly lean of peak due to uneven mixture distribution. This sort of uneven mixture distribution is common in carbureted engines, but there are many fuel injected engines which also have relatively poor fuel distribution. In this case, a set of custom, balanced fuel injectors are required for your aircraft if the engine is to run smoothly lean of peak.”