What Weight Gear Oil for a County Line Rotary Cutter Gear oil is a lubricant made specifically for transmissions, transfer cases, and differentials in automobiles, trucks, and other machinery. It is of a high viscosity and usually contains organosulfur compounds. Some modernistic automated transaxles (integrated transmission and differential) do non use a heavy oil at all but lubricate with the lower viscosity hydraulic fluid, which is bachelor at force per unit area within the automatic transmission. Gear oils account for about twenty% of the lubricant market. [ane] Most lubricants for manual gearboxes and differentials contain extreme pressure level (EP) additives and antiwear additives to cope with the sliding action of hypoid bevel gears. Typical additives include dithiocarbamate derivatives and sulfur-treated organic compounds ("sulfurized hydrocarbons"). [2]