Before going deeper in the discussion of flying 'too lean' it is best for everybody to uderstand the basics.
Have a look at this:
<a href="http://www.pplir.org/renaissance.htm" target="_blank">http://www.pplir.org/renaissance.htm</a>
I fly lean of peak on an IO520 since about 4 years now, properly equipped with 6 cylinder engine analyser, GAMIjectors and proper understanding of what's going on. Can't hurt the engine by running lean of peak in cruise.
If the engine won't run smooth lean of peak, then the mixture is not properly distributed (most carburetted engines), there is an induction leak, or the injectors are mismatched to the airflow of the engine.