The .46 LA is a great sport engine that is very easy to tune. It will also deliver plenty of power for a typical .40-size airframe as long as you're not attempting 3D-style hovering manuevers.

A plain bearing engine like the .46 LA will run best on fuel with castor oil or castor/synthetic blend lubrication. The castor oil will help seal up the crank case and provide for the smoothest operation of the engine. A fuel blended with 100% synthetic lubrication won't work as well.

The .46 LA should fly the Phoenix Extra 300S just fine. You might also consider the Magnum XLS .46A 2-stroke, which is very similarly priced to the .46 LA. It will be a little more finicky to tune, but should produce more power.