Joey,


The fuel you have sounds like PowerMaster GMA 5%.

It is a good fuel for this engine.

The Senior Telemaster is a pretty slow flying plane.
With a 13x6 prop you would have the engine spinning more slowly and making somewhat more static and low-speed thrust.

It is better for the big Telemaster, than a 12x6.
The smaller prop, due to spinning faster, has a pitch-speed ~15% higher and the model may dive a bit faster, but will lose speed when you try to climb.

The bigger prop will also make the take-off run shorter.