PAW have been recommending the use of pure medicinal castor oil (100% British Pharmaceutical [BP] grade) as a lubricant for their engines since my father were't lad.

The first sentence in paragraph 5. and the table in paragraph 14. in the instructions referenced below tell you all you need to know about it's suitability as a two stroke model engine lubricant:

http://www.eifflaender.com/instruct.htm