Hi,
The way I see it, it´s important to ensure proper cooling and adequate mixture management.
low power settings combined with speeds a bit higher than best range, shouldn´t be a problem, because heat generation is proportional to cooling drag generation
(besides you´re low, air is dense and cooling drag is important, if you were to fly at low power settings at higher altitudes that would be more of a problem, because even tho temp decreases, air density and cooling drag decrease dramatically).
Now, if the airplane is heavily loaded, and you ´re told to maintain constant power settings as opposed to constant speeds, you might overheat your engine, because you´re flying a lot slower and producing a lot of heat.
I fly a baron, and sometimes I use low (45% bhp) power settings but cruise at the fastest speed that setting gives me...ensuring enough engine cooling...and cycle the throttles thru a higher power setting and back once every 20 min or so, to avoid constant power settings for long.
Remember that the same power setting can give you diff speeds in level flight..one corresponding to the 1st regime of flight, and the other one in the back side of the power curve (2nd regime) THAT is the one you want to stay away from.
Mixture management and cowl flap operation are very important too. You don´t want your engine too cold either.
Most engines' mixture controls are supposed to be left alone at the full rich position from 0 to 3000 feet. I normally lean them just a bit closely monitoring EGT and CHT indicators.
The combination you want to avoid is heavy, slow and too lean. If you stay away from that, then TBO should be the one predicted by the manufacturer.
Hope this helps,
SW
www.patagonianskies.com.ar