Timble is correct to some extent. I keep reading posts about 'medicinal castor oil' which are incorrect. The castor you buy in a pharmacy should be labelled Castor Oil, USP. The USP designation means it is described in the United States Pharmacopeia, a set of standards recognized around the world. The description SPECIFIES that 'It contains no added substances'. It is not polymerized, hydrogenated, dewaxed or bombarded with nuclear radiation. So if you buy castor in a pharmacy, just read the label. If there are other ingredients, don't buy it. It is fine for human consumption for it's intended purpose which is as a laxative.

As far as it's suitability for model engines, the battle will go on forever. There are those who love it, those who hate it and they will never change their opinions. Castor is often associated with the 'old guys'. Like me. When you've used it for 40 years you get comfortable with it. I bring all of my fuels up to 20% TOTAL oil with castor the amount depending on what's labelled as being in the fuel. If the fuel label doesn't specify the total oil content I don't buy it. And I can buy Sig castor or WalMart Castor oil USP a lot easier than I can find some proprietary synthetic with some sexy name like Acme 1207Xalpha. And my bottom line is that I have engines older than most people on this forum that still run like new except for that Saito 1.80 that I buried in a smoking hole in the ground 2 years ago. Even castor couldn't save that one