Running-in an engine properly will insure it will give you a good service life with the power it's designed to have.

The initial run-in is intended to get the engine to the point where you can fly it as it continues to break in. Your O.S. Max 1.40 RX will break in over the first gallon or two of fuel. If you rush it, the service life and power will be reduced.

Once the initial running is done, the model can be flown with the engine at a rich setting. You'll gradually lean it to peak RPM over a series of flights. At some point, you'll be able to lean the engine to just rich of peak RPM and leave the needle there.

When running on the ground, the model is just about the worst place to do it, unless you can put the model on a stand or table a few feet up. Otherwise, the engine will suck in a lot of dirt and trash, which will eventually wear out the engine. Even on grass, you'd be surprised how much dirt can go through an engine.

Just follow the instructions that came with the engine, and you'll be fine. It's an F3A engine, and not a general sport-flying engine.