Bingo, bingo! Couldn't help myself. I suggest more oil because the rod bearing loads are fairly high in a lugging mode, while the oil film is thinner. And for the record, the RPM's that the retired Coastie uses are not lugging, but he is going to be way down in power. He makes up for it with a larger engine, giving enough torque to turn a bigger prop.

Mesa, I too like to fly at lower power setting part of the time. This mode of operation is where a ring'ed engine is best. On the other hand if you enjoy boring holes though the sky at higher speeds, the AAC, ABC, ABN style of engine works better.

On that size of engine (35-45), I usually target 12-13K. You can often improve the performance of any model by doing a lot of prop experiments. If you enjoy the vertical possibilities, the larger diameter and lower pitch adds climbing ability. I usually drop pitch first, so instead of a 10x6, a 10x5 might add quite a bit more performance. While you lose some speed due to the lower pitch, you may gain more than enough rpm to fly even faster due to the increased rpm and power. Or you might go from the 10x6 to 11x5 and find the additional climb and lower landing speeds meet your needs.

There are really no hard fast rules here, every model and engine combination requires a half dozen to dozen flights with different props sizes to find the one that works best. Even when you get it down to one or two that work well, changing prop brands in the same sizes may improve things. Or not. Kind of like a trip to the eye doctor when he has you comparing lenses. Also the conditions make a fairly big difference. What works well at sea level might not be so great when you take a trip to the mountains. Hot days Vs cold days, etc.

The main thing is to experiment. And that's just the engine, throw in the engine thrust line adjustments, models balance and throws, and general trimming it may take 30 flights just to get an acceptable flying model. Anyone who has picked this as a hobby will have a totally open ended commitment to new things to learn.