That's overkill for this engine.

You only need 5% nitro AT MOST on the .61 ST engine unless you are flying at higher altitudes.

Also higher oil content is needed by smaller engines. The 18% often cited is applicable to small .25-.40 engines, not larger ones, but that mantra has been SOOO often repeated that it is assumed to be true accross the board even by "old timers".

As you go up in size because of the relative change in the size of the area of moving parts, less oil per volume is actually beneficial and results in more power.

( There were several indepth articles by the fuel experts, Greke and others, on this last year in RC Flyer and other magazines where this is covered in great detail.. ).

In a larger engine you are loosing performance due to the higher oil in the fuel. Since 18% is a catchall provided by most fuels, you'll be fine with Cool Power or anything similiar. Synth fuels are more "slippery" than Castor so you frequently find the oil percentage a TINY bit lower... e.g. 17% instead of 18%, but this does not harm most "normal" engines... however Synth have a lower flash point, so you don't want to run a hot engine on Synth fuel...

Fortunately the ST's run very cool no matter what you do to them it seems!

I use 5% "Sport" Cool Power on all of my ST's, and they show no visible wear after years of use... plus there is no Caster buildup anywhere.... ( though Castor has some very nice properties, especially useful on brand new engines and engines that run hot... ).

Save the difference and stick with the lower Nitro on the ST .61, especially if it is a used or already broken in engine.