Wow, so many misconceptions in a single thread. Where to start???

First off, to generate power in a 2 cycle model engine, it requires RPM. OS recommends rather heavy props for their AX line, which I think is more marketing than reality. Not to say that you can't run a 13x7 prop, but other than being quiet there is nothing to recommend this approach. With a heavy load, the main needle valve becomes quite critical with a very narrow range between rich and lean. And since the rpm will be very very low (<10,000), you will not be making very much power, certainly far less than 1.75 PS it is rated at when at 16,000 rpm. At 10K or less I doubt the actual power output would be above 1.0 horsepower.

As to the amount of oil exhausted and the effects of RPM, most of the oil content is for the rod bushings, not the piston and sleeve. As you run an engine at lower rpm, the rod actually need more oil than an engine that runs at a much greater rpm.

While a 11x5 APC does sound a bit light for the engine, it will not harm the engine or shorten it's life in any measurable way, as long as it not run lean. But because the needle will be so much broader, it will be very easy to needle for a successful flight. The 12X5 APC prop would be my pick along with an 11x7, depending on airplane drag.

I would suggest those that normally run their AX 55 with higher loads to experiment a bit more with lighter props that get the rpm into the 12K+ range and report back their findings on the airplanes performance.