"mm²/s" is the scientific unit for what is called "kinematic viscosity". The unit "mm²/s" is also called centi-Stokes (after the scientist/engineer), and you'll see the unit cSt in tables of data for oils etc. The full SI unit is the "Stokes" but it is a rather big unit so the centi-Stokes is commonly used.   The viscosity of a liquid (or fluid) is the resistance to flow in simple terms. A "thicker" fluid has a higher viscosity. There are 2 ways to look at viscosity from an engine