Myths in aviation
Oversquare is one of the great myths of aviation. If an engine blew up 'oversquare' no turbocharged engines would ever work. The numbers are just that - numbers. There is not relationship mechanically between RPM and MP.. if you measured MP in mmhg instead of inches of mercury maybe this myth might never have started.
The things that destroy engines are excessive temperatures and pressures (which leads to detonation). When operating larger engines, care must be taken to aviod areas of high temps and pressures which occur when operating at high power settings (above 60% power). Keep the engine cool and power settings low and you can be safe to operate the engine anywhere in the permitted RPM/MP limits which will be defined in the POH. For higher settings use the specific settings which are in the POH, but be careful - for some engines (Turbo'd Lycomings - TIO-540) the recommendation for 'best power' puts the engine in the worst possible operating conditions (50 degrees rich of peak EGT - where the peak cylinder temps and pressures are).
As for reducing RPM to increase range - it works. At a lower RPM there is less frictional loss inside the engine as it is turning slower. That being said, you cannot do this at high power (>60%)settings as you are increasing the peak cylinder pressures so reducing the engines detonation margin.
Bevan..