Gas! Engines

For Cox and most other .049 engines you should use oil with at least 1/2 castor oil & 1/2 synthetic oil. 20%-25% is good. Most Cox engines run fine on 15% nitro, but 25% nitro is what would be recommended mostly. More nitro than that is really just a waste for general sport flying.
Larger engines which have ball bearings need less oil- probably around 15%-18% and don't really need castor oil, although adding 2%-3% castor oil is good to be safe. Older engines (.09 and larger) with crankshaft bushings and iron pistons should use castor oil also. Around 20% total is good with at least 1/2 of that castor oil also. Many of these engines don't need nitro but 5% will help for better running.
This is just my personal experience. Everyone will have different experiences and some of the percentages will vary depending on their opinion.

Rod.

I agree with Rod on this one but I would add that for diesels - only use castor oil.

For diesels

Castor oil

There have been many fuel formulas, with oil content ranging from 12% - for speed TBR engines to 33% - a British norm.

In the typical diesel, with an iron (Meehanite) piston and hardened steel cylinder, the minimum oil content I would recommended is 25%. It's a characteristic for proper lubrication of those two materials. A safe bet is 25% oil for any engine. That way there's lots of lubrication for the connecting rod. PAW recommends 30% for break-in, and it isn't going to do any harm to run higher oil (over 25%) all the time, but the surplus is going to go out the exhaust and may not contribute much to lubrication beyond 25%. and is messy.

There is greater force on the con rod bearings of a diesel than of a glow engine, so the insurance of the castor oil is well worth the added mess from the exhaust spray.

Ether

A typical ether content is 33%. This assures good atomisation, and also gives a safety margin for loss due to evaporation, for the ether will evaporate quickly if the container is left unsealed (especially when the air is hot). Engines will run on lower percentages but don't go lower than 25%. At some point the fuel will not atomize as finely and power can be expected to drop. There might also be some effect on combustion, and it is necessary to increase the compression which isn't a good idea as it strains the moving parts.

Some fuels are even higher than 33% in ether. They will burn cooler than "standard" fuel and will produce less power, but there may be some times when this is an advantage, as with a Davis head for a Cox engine; the lower temperature is not as likely to melt the Teflon disc that seals the top of the cylinder. This disc melts when "normal" fuel is used and the engine is tuned for full power. With a teflon disc use 40% ether.

Ignition improver

Ignition improver, like isopropyl nitrate, helps the ether to ignite at a lower compression ratio, and also smooths the combustion process. Without it, a diesel has a crackly, rattly sound, and compression has to be set higher, which puts more load on the moving parts.
With it, a diesel runs a little more smoothly and purrs like a tiger (rather than barking like a dog!). A typical mix is 2% for engines up to about 3.5cc and 1.5% for larger engines.

Kerosene/Paraffin

This makes up the balance but I have seen all sorts of substitutes and the engine still runs! Why substitutes? I don't know! Paraffin is cheap!

Everyone has their own favourite mixes and sometimes it is engine specific, Almost always I use 33% ether, 33% castor oil, 33% paraffin and 1% ignition improver. I like my engines to bark like dogs!

Friends, thanks for such good information, very useful !! Thanks again, greetings, Mauricio.-