Gear oil is a heavy, foul-smelling lubricant designed for manual transmissions, differentials, and transfer cases. It has a much higher viscosity (thickness) than engine oil, typically rated around 75W-90. This thickness ensures it clings to gear teeth under extreme pressure and high torque loads without being squeezed out. It contains sulfur-phosphorus additives to protect against metal-to-metal contact (scuffing). Using engine oil instead of gear oil in these components will lead to rapid gear wear and whining noises.