[Zendalar]

Black is synthetic rubber (can't take gasoline or oils very well)
Black o-rings tend to harden when they age, and these are usually seen in carburators, even though they can't handle the gasoline very well. These are the cheapest o-rings.

Red is real rubber.

blue is silicone (good for high temperature, not soluble in gasoline or oils, can handle friction very well without breaking -> When pressure is released it should get back its original form).
Blue o-rings, are used to seal case halves in combination with those "case-through-bolts" (sorry, dont remember the word). When for example Porsche engine is taken apart these are usually to be found still very flexible.

I use sealing glue with paper gaskets, this is lacquer like material which binds the paper gasket between the materials. I have been using gasket-lacquer in carburator gaskets with very good results (with pure paper gaskets it leaked, no matter how thick the paper)...the name of the lacquer was ...Green something... =)

EDIT the EDIT: Yes, usually I oil all o-rings disregarding what material they are made of.