Coming from a question about [distances between satellites in GEO][1], what liquid propulsion systems do a majority of GEO satellites carry to maintain their slot positions? It is easy to find information about Merlin engines, SSME, Rocketdyne F-1, etc., but smaller purpose engines are harder to find.

**UPDATE**

Thanks for the info about the propellents used. What about the engines?
  [1]: http://space.stackexchange.com/questions/2515/how-closely-spaced-are-satellites-at-geo