A word of caution on that castor oil mixture: castor oil is not really compatible with petrol, and will not mix reliably with it. It isn't really a great idea to use such a mixture in any 2 stroke, but because it isn't likely to reach high temperatures in an unmodified lawnmower engine, there probably won't be much varnish formation. You can use castor oil as lubricant in an engine fueled by straight methanol - castor oil is compatible with that. Of course the methanol is obnoxious in several other respects, but at least it will mix with castor oil.
I rather like the smell of burned castor oil myself, but the safest way to enjoy it while mowing is to put a very small amount of castor oil in the straight petrol fuel of a 4 stroke lawnmower, and keep its sump filled with an automotive-rated mineral oil. I've known vintage racing car enthusiasts to adopt that solution, with the support of an oil company (the oil company tech rep advised them never to put it in the sump, however, if they were running the engine on petrol.)
