The oil that is mixed with a nitro fuel is what helps lubricate all the internal moving parts as well as keep the engine running cool, because if a nitro engine gets too hot not only will you see a decline in performance, worse can happen. You could permanently damage or ruin the engine. Nitro fuel normally has both castor and synthetic oils in a predefined mixture set by the company that makes it. What percentage of both are mixed with the nitro fuel is not typically disclosed (although it might be -- read the container label). The percentage of oil might range anywhere from 8% to 25% with 15%-20% being the typical amount of oil found in nitro fuel.
Manufacturer Recommended Fuel Percentages
When choosing nitro fuel percentages it is best that you first look in the manual that came with your nitro RC. Check to see what percentage is recommended for your specific nitro RC. If the manual doesn't list what percentage of nitro to use, it would be a good idea to ask your local hobby shop what they think, because most of the employees in a hobby shop either have worked on or own RCs and they also have ways of getting the info you may need. There are many different brands of nitro RC fuels out there. There is no consensus as to which one works best -- it's more like trial and error to find out which one works best for your nitro RC.
Engine Size and Nitro Fuel Percentages
One thing to keep in mind (beyond the advice in the manual) is that the Nitro RC engine size does matter when deciding what percentage of nitro to go with.
- Engines that are .12 - .18 generally use 10% to 20% nitro fuel
- Engines that are .21 - .26 or .27 generally use higher percentage nitro fuel in the ranges of 20% to 40%.
Changing Nitro Percentages
Back to the original question about switching nitro fuels, is it OK? The best answer: maybe.
I have switched fuels in a pinch when I've run out of what I normally use while I'm doing some backyard bashing with friends as long as it was a small change -- 5% or so. Going from 10% up to 20% can be harmful if your nitro engine is not tuned right for it (leaning out the air/fuel mixture). Going from 20% down to 10% will give you less in performance and you will probably need to do some additional tuning (richening the air/fuel mixture). But in general, small changes won't usually damage the engine if you pay close attention to how your RC is running and tune the engine as necessary. Avoid making sudden switches to much higher or lower percentage fuels than you normally use and you don't want to be constantly switching back and forth.
Ideally you should stick with the same brand of nitro fuel even if the nitromethane percentage is different. Each brand may use different types or percentages of oil and other additives so you don't normally want to be switching brands and nitromethane percentages at the same time.
The bottom line is that switching nitro fuel is a matter of trial and error. It's best not to do so if you are new to nitro RCs. Changing fuels will almost always require re-tuning your engine.