I was under the impression that castor oil would alter the elastic properties of the diaphragm. But YS allows up to 2% Castor so I may be off base in my assumption.
The YS factory recommends no less than 20% oil in the fuel mixture. Many commercial fuels contain less than this. Use of these fuels may cause damage to the engine and/or poor running characteristics. Not to mention invalidate the guarantee. Fuels containing large percentages of castor oil can cause fuel pump/regulator trouble. We recommend you use 100% synthetic oil in the fuel, but 18% synthetic and 2% castor is suitable also.
The following website has some good info on setting YS Engines
http://www.julton.net/Fact%20Sheet%20YS%20Engines.html