It is a high viscosity oil containing organosulfur compounds. An important fact: motor oil viscosity and gear oil viscosity use two different API ratings systems; the 75W-90 gear oil rating is not a thickness relative to motor oil weights.
Most oils for manual transmissions and differentials also contain anti-wear and extreme pressure (EP) additives to cope with the sliding motion of hypoid bevel gears and, generally, the higher the rating the more EP that is present in the formulation. EP additives also contain phosphorous/sulfur compounds, which are corrosive to yellow metals such as the copper and brass used in bushings and syncros.