Nitro fuel for RC cars contains a mixture of nitro-methane and oil. The fuel is classed by the percentage content of nitro-methane so 15% nitro fuel contains 15% nitro-methane and then 85% oil and other ingredients. In general a lower nitro content makes starting an engine easier and is also kinder on the engine as it contains more lubricating oil. But a higher nitro content can give better performance. Hobbyists will usually start a new engine on a lower nitro content and then when it is well run in start to use a higher content. It is important to note though that some size engines will not work with some percentages, a smaller engine will require a lower fuel content and 21 size engines and bigger will require higher contents. For normal use we recommend around 15% nitro for 06 - 18 size engines and around 25% for bigger engines.