Castor oil gets thinner up to about 300 degrees and then starts to get thicker at the higher temperature where other oils are starting to loose their lubricating properties. Castor is able to lubricate well up to the temperature just before it turns to ash. The goo or brown varnish from castor is still a good lubricant at extremely high temperatures.
Castor will still make good power in oil ratios at 5:1 when properly tuned
Most of the synthetics I have tested will not make good power at ratios richer than about 30:1 when optimally tuned.
The high ignition temperature of the synthetics seems to hinders the initiation of the combustion process when used richer than about 30:1. The castor oil's lower ignition temperature does not seem to hinder the combustion process or the power. Some of the small model airplane fuels contain as much as 20% castor. 20% oil is a 5:1 ratio.
You will never hurt a two stroke engine by using a lot of oil in the fuel when properly tuned.