Heavy Oil
From Captain of Industry Wiki
Heavy Oil is a petrochemical liquid product produced when processing Crude Oil in the Distillation (Stage I). In the early game, the only useful application for Heavy Oil is its burning in a Boiler (Gas) to generate Steam Hi, which can then fully supply two Distillation (Stage I), two Distillation (Stage II), two Distillation (Stage III), and one Sour Water Stripper machines with a 5% excess leftover.
In the mid- and late-game, Heavy Oil is sometimes combined with elemental Hydrogen in a Cracking Unit to produce Diesel and Fuel Gas.