leaning
We operated a fleet of single engine (lycoming) aircraft for many years. We started operations in 1988 and initially left the choice of leaning to the pilots. After a few years, we found several cracked cylinder heads, probably due to leaning and forgetting to readjust during the descent. I decided then that the extra fuel burn was less expensive than replacement heads so instructed pilots to operate in full rich throughout flights. We had no more cylinder head cracks. We did not suffer from plug fouling - plugs were cleaned and checked at 50 hours. I must admit that I did lean on longer flights where fuel economy became important.