The thickest motor oil readily available on the market is 40W70. This oil is made for antique cars with diesel engines that burn oil excessively.
The thickness of oil is more commonly referred to as viscosity grade. The higher the number, the more resistance the oil exhibits. Due to the addition of modern polymers, multi-weight oils are common. In the case of the 40W70 oil, the number "40" indicates the viscosity while the motor is hot during operation, and "70" indicates the viscosity while not in operation. The high operational viscosity helps prevent it from seeping through valve guides and burning in the combustion chamber. This is not possible with single-weight oil.