Oil Tar

Oil Tar is a petroleum product produced as a byproduct from fractionating, and from solidifying some heavier oils. It has 3 variants, that are all interchangable in crafting: Oil Tar, Crack Oil Tar, and Coal Tar.

Production
Crude Oil can be turned directly into Oil tar with the Industrial Solidification Machine, which is useful early on in the oil stage, but will become a waste later on. More Usefully, Heavy petroleum Products such as Bitumen and Industrial Oil can be solidified instead, leaving you with the lighter petroleum products to be used elsewhere.

Alternatively, Crack Oil Tar can be produced passively as a product of fractionating Cracked Oil in the Oil Refinery, replacing sulfur as a byproduct when refining regular Crude Oil. Crack Oil Tar can also be solidified from Cracked Oil like regular Oil Tar, but this is a waste as you can simply solidify the Crude Oil, instead of having to crack it first.

Coal Tar is only produced from solidifying Coal Oil, which is only produced from liquefying Coal. This is the easiest way to produce Tar in large quanities, but since the Industrial Liquefaction Machine requires Tar to craft, another method may be preferred for your first bits of tar.

Uses
Oil tar can be used as a black dye in various crafting recipes, but its main use is in crafting Fluid Tanks, such as Steel Barrels, Tanks and the BAT-9000.

Trivia

 * The 3 Tars' textures are all recolours of Orange Dye.
 * IRL, what would be considered Oil Tar is much more of a liquid than would appear Ingame.