Most (but not all) 2-stroke engines do well on 18% oil, and engine manufacturers recommend this minimum level of lubrication for a very good reason. In order to reduce costs and maximize profits, many fuel companies are cutting the oil level and/or quality of the oil in their formulations. Not good. Castor oil is a great oil for 2-stroke engines and has been around a LONG time. It is a natural product and should be poured up and immediately capped and put back in a cool dry place, room temperature at the bottom of a closet is fine. I personally use 2-4% castor in my 4-stroke engines, but higher castor gums them up pretty much, so keep the castor low in 4-strokes. Here is a little information about castor oil. For model fuels, AA grade castor is fine, highly purified castor and castor with additives, like at the pharmacy, is less good for model engines.

Castor Oil is a natural oil obtained from the seed of the castor plant. It is unique among all fats and oils in that it is the only commercially important oil composed of approximately 90% of a hydroxy, unsaturated C18 fatty acid-ricinoleic acid.

Chemically, Castor Oil is a triglyceride (ester) of fatty acids. Approximately 90% of the fatty acid content is ricinoleic acid, an 18-carbon acid having a double bond in the 9-10 position and a hydroxyl group on the 12th carbon. This combination of hydroxyl group and unsaturation occurs only in Castor Oil. The primary sources of Castor Oil are from India, China and Brazil. Scientific and historical records reveal that regardless of origin, the chemical nature and composition are remarkable uniform.

There are two technical grades of Castor Oil: (AA grade castor shown here_:

Pale Pressed, also known as AA Standard, is obtained from the first pressing of the castor bean. Pale Pressed Castor Oil is lighter in color and lower in acidity. This is the one for your model engines.

Note added: Many modelers have an aversion to some castor film on their airplane after a flight, but I maintain this is good, it is the same stuff that is lubricating your engine well, especially sticking to the connecting rod and lower end of your engine, and yes, castor oil does reduce rust in your engine. Castor forms long chains during lean runs and further protects your engine and is not burned up like many synthetic oils. I personally use a blend of castor and synthetic oil for all of my flying and use only 10-15% nitromethane for most of my engines. The large ST engines will use 10-12% total oil and 5% nitromethane is plenty. The old style Cox engines still run well on 20-30% nitromethane, but with just a castor based fuel. Control line engines still generally run well on 20% oil fuels. For those who remember the old Fox .35, or better yet, still run one, 25-29% castor is fine. The old SuperFuel was about 29% castor and 5% nitromethane. Again, for Americans, in general, too much nitromethane and not quite enough oil is the rule. I have seen many an engine ruined by too much nitromethane, too lean a run, and too little oil, but seldom have ever seen an engine ruined by running too much oil. Maybe a very dirty plug and that is about it. That should tell you something.