IMHO, you can safely use your synthetic fuel without worry.
In Sweden, where I live, most people use synthetic oils exclusively with no castor oil addition, be it small or big engines.

Unless you run your engine lean for extended periods, the chances of damaging your engine approaches zero.

Irrespectively of the oil used, when using high nitro fuels it is always a good idea to use some kind of after-run oil in order to prevent corrosion.

/Red B.