A fuel with 10 to 15% nitro is fine, however higher nitro content (up to 30%) is suggested by Horizon Hobby to be OK (see also the section ‘Special Tricks’). More nitro does provide more power but the
fuel cost does substantially increase with higher nitro.
Nitro content greater than 15% is not necessary. If you can afford to buy fuel with 20-30% nitro a bit more power is available, however you might be better off buying a larger engine, or a lighter model, if you
are that desperate for more power.
The oil content recommended by Saito and Horizon Hobby is 20%. Horizon Hobby state that oil that is a blend of castor and synthetic is acceptable. Saito instructions (that no longer accompanies engines sold in
the US) say the following: “Since the four-stroke engine has high exhaust temperature and carbon is apt to accumulate when castor oil type lubricants are used, avoid using fuel containing them.” This
seems to imply Saito recommends fuel with no castor at all.
It is not clear that 20% total oil content is really needed as 15-18% seems to work fine and is common with commercial fuels labeled for 4-stroke engines. It is really also unclear that all-synthetic, or a
blend of castor-synthetic, is preferred; both are probably OK, however no more than 5% castor with the remainder synthetic oil is usually recommended. Byron 4-stroke fuel has a total oil content of 16%
with about 3% castor and the remainder synthetic.
I have heard from some European Saito flyers who are mixing their own fuel with 10% total oil content. No adverse effects have been reported. However 15% to 20 % is a safer blend.
In summary, any commercial 2-stroke or 4-stroke fuel should be fine, provided you make sure the castor oil content is low, with mostly synthetic oil. The only difference between commercial 2-stroke fuels and
4-stroke fuels is the total oil content (4-stroke fuels typically have 2-3 % less total oil content compared to 2-stroke fuels).