A petroleum base stock consists of many different oil fractions that form the final product. Generally, molecules of a petroleum base stock are long carbon chains that can be sensitive to the stress of heat and “boil off” at relatively low temperatures. Engine temperatures break down these molecular chains, changing the physical properties (such as viscosity) of the motor oil.
The difference with synthetic base stocks is that molecules are uniformly shaped, which makes them more resistant to the stress of heat. Because AMSOIL synthetic motor oils possess these uniformly-shaped molecules they have a low “boil off” rate thus, their physical properties (such as viscosity) do not change.