I generally don't use less than 18% oil in midsize glow engines for aircraft. And I like fuel that uses a blend of 20% caster with the rest synthetic. Not all fuel manufacturers measure the oil and nitro content the same way. The proper and honest method is by volume, but some are giving percentages measured by weight. Since both nitro and oils are heavier and more expensive than methanol you get less of each when done by weight. There is not a signicant difference in nitro between 5 to 15%, but lowering the nitro slightly is the same as richening the engine slightly. Some engine may have slightly higher compression and do poorly with higher nitro fuels, it a bit like the timing on your car, but usually a change in glow plug is all that is required. Sometime it can take adding a shim to the head with some engines. When you start to go to the performance levels of nitro in the 25 to 70% range, things start to get very difficult in engine set-ups, and very few modelers go there anymore. Adding a tuned pipe also complicates engine set-up and tuning. As it is, with the move to electric, there are less people honing their engine skills.