I was unfortunate to have had an event leading to the failure of one piston and subsequent engine shut down on a PA-31. Data on our engine handling technique was used to compare to those used by Whyalla Airlines (who had an accident in 2000 which killed 8 people in a PA-31) to establish if engine leaning was a contributing factor. The report can be found at
http://www.atsb.gov.au/aviation/occu...il.cfm?ID=317. It seems that Whyalla's double engine failure was initially a failure of the crankshaft of one engine, and the second engine, having had a company endorsed history of having been operated lean, holed a piston and led to that engines demise.
These high power engines are very sensitive to lean engine handling (all operators investigated were operating engines within engine manufacturer's recommendations). Unless comprehensive engine analysis instruments (ie: JPI 6 cylinder egt, etc) are used, using a single probe egt and leaning beyond peak egt is foolhardy, just to save a couple of litres of fuel per hour.