Crude Oil

Crude oil is a very important liquid resource. Known as "black gold", it indeed is quite useful, but not in itself actually. Crude oil has little uses in its natural state, only solidification, which is a waste. To get any real uses out of it, you have to refine it into better and better petroleum products. It is entirely black in color.

Obtention
All petroleum products come from oil deposits deep in the ground and must be detected via an Oil Reservoir Detector. Oil deposits can be mined up, but they no longer can be used as a type of single-use fluid transporter.

An oil derrick or pumpjack directly over the deposit must be used to suck up all the crude oil.

Uses
Crude oil can be solidified, but that's a waste.

It must be boiled and then put into an Oil Refinery to make actual use of it.

This will produce Heavy Oil, Light Oil, Naphtha, Petroleum Gas, and Sulfur as a byproduct.

Trivia

 * Crude oil is not renewable and has no other means of acquisition once your world runs out of oil deposits.
 * Many petroleum products can be produced via coal liquefaction.