I’m not a fan of castor oil at all. I use strictly synthetic in all my engines. The argument is that in the event of a lean engine run the castor oil gives better protection. My view is that there is a higher potential for a lean run using castor as it leaves deposits. Many fuel formulations contain both but the castor content is typically not more then 5%. So where does that leave you? My suggestion is that if you feel you need to cut your fuel is to order both Klotz synthetic oil and castor oil from Sig Manufacturing and mix your fuel so that you have an oil content of 20%, 15% synthetic and 5% castor.