Here is an excerpt from Wilcat Fuel site:
Wildcat recommends the lubrication package be totally synthetic. The use of synthetic oil is important in four stroke engines to prevent gumming and varnishing of the lifters and coking of the valves. Varnishing will occur on castor based fuels. This can lead to sticking valves, which will cause the valve set to get knocked out of time. Coking of the valves will cause improper seating of the valves causing a reduction in compression and incomplete combustion. Four stroke engines also have high exhaust gas temperatures and the use of synthetics greatly reduces carboning at the exhaust ports.
Read more about it here:
http://www.wildcatfuel.com/fuel_24cycle.html