It takes a HUGE overdose of Castor Oil to be dangerous. Toxcity studies funded by the US National Institutes of Health in the 1970's showed no signficant toxity in animals receiving up 10% castor oil (by weight) of their food for 13 weeks. see http://ntp.niehs.nih.gov/ntp/htdocs/ST_rpts/tox012.pdf
The estimated toxic dose of Castor Oil in humans is believed to be on the order of 1 liter (1 quart).
And yes, Castor is an extremely effective in 60-90ml doses.
There is some debate as to whether or not doses larger than about 60ml make any difference.
Once the digestive enzymes break down enough of the oil in Ricinoleic acid, and produces the ricinolate compounds that are responsible for the action, the effects are sufficiently strong that any oil that hasn't been broken down will be expelled before it can broken down.
Castor oil is essentially inert until the ricinoleic acid is released.
While Castor Beans contain an extremely toxic compound, Ricin, the processing of the beans is carried out under conditions that very effectively destroy the toxin.