It's not really the trainer which needs fuel - it's the engine in your trainer. Which engine do you have? If it's a new engine then it's likely built with ABC or ABN metallurgy. Those designations merely mean the piston cylinder is brass (the "B" part) which is either chrome (the "C" part) or nickle (the "N" part) plated and an all aluminum piston (the "A" part). The crankshaft is supported either by a bushed bronze bearing or by twin ball bearings. These engines normally don't need a full 20% oil but that quantity is ideal for breaking in an engine and it's early flights.

The nitromethane component adds a bit more power (and a lot more price) to a gallon of fuel. Most engines with higher compression run better on lower nitro content. Most overseas flyers use zero nitro. The 5% is a fine sport fuel while 10% will aid your idle and make the needle valve a little less critical to set. The vast majority of fuel sold however, is 15%. The 10% should be a good compromise.

Four stroke engines seem to need more nitro content.

NEVER use any fuel with less than 18% total oil content unless it's fully broken in and you understand how to set your needle valve properly. An engine should never be leaned out to a screaming 2 cycle setting on the ground.

I fly my trainer with a Thunder Tiger .46 Pro engine - an ABN ball bearing engine. I use a fuel with 18% total oil (including castor) and 10% or 15% nitro. The higher nitro did seem to help to reduce deadsticks.

Sig makes a very high quality fuel.