Tetralubisol was a thin, low viscosity, milky fluid in use as a lubricant about starships in the late 23rd century. It was quite poisonous; a dose of as little as a few milliliters was sufficient to cause symptoms, including severe asphyxia and unconsciousness.
In 2266, Lenore Karidian attempted to murder Kevin Riley by poisoning him with this substance. Doctor McCoy was able to save Riley's life, although it was touch and go at first. (TOS: "The Conscience of the King")