Engine wear and fuel savings can be achieved by reducing RPM and increasing manifold air pressure (MAP) to maintain horsepower. Modern engines have approved power ranges for which this practice is allowed. These ranges are in the POH but can also be found in the engine manufacturer's handbook.
AVweb article: Why over square is good for your engine.
Excerpt from the document:
Following manufacturers recommendations, turbocharged engines routinely are allowed to operate at 8, 12, even 15 inches "over square" depending on model. (Think of it: a foot over square!) Normally aspirated engines don't have sufficient manifold pressure to do this, but most of them are permitted to operate 1 to 4 inches "over square". (Check the cruise performance charts in your POH.)
There are a lot of technical reasons why high MP/low RPM operation is the optimal way to operate your engine in cruise. But before I discuss them, let me offer an automotive analogy.
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