David,
Last time I checked - not recent - KLOTZ offered two synthetics: KL-100 and KL-200.
I forget which, but one of them has 25% castor, the remainder synthetic. The other is all-synthetic.
For a Fox 35, iron piston in steel sleeve, after a proper half-hour or so break-in, SIG 10/25(all castor) should be just fine. Much more time on all-castor fuels and you may start building some varnish on the piston. That's not necessarily a bad thing. What could be bad is having a synthetic part of the fuel dissolve and flush that varnish out. The gunk isn't pretty, but it helps the piston seal and last. "Deterge-ing" it out may return the engine condition to a much earlier stage in the break-in process.
Jim T or Doc H has mentioned running Fox 35s on synthetic including oils at around 22% total from first run on. That becomes the break-in condition, and they say they've had no problems doing that. Switching in midstream is also bad on horseback...