Ed,


Please note what Webster's has to say [link=http://www.webster-dictionary.org/definition/Caster]about caster[/link].

...and [link=http://www.webster-dictionary.org/definition/Castor]about Castor[/link].


Strangely, castor can be used instead of caster, but not vice versa.

In any case, using 'caster' to signify 'Castor oil' would be incorrect.