Castor is supposed to protect against a lean run, but it gums up the engine after awhile with varnish build up. Helis typically run richer than planes, so Heli fuels usually go with all synthetic in favor of less varnish build up, since lean runs should be less likely. Most plane fuels still have a little castor (2% or so) in addition to synthetic to protect against a lean run. As long as you're careful not to run lean to often or too long, the all synthetic heli fuel will be fine. Total oil content for heli and plane fuel is usually the same (17-20%). Some heli fuels may have a couple percent more than the plane version for some brands.