ISO VG 680, AGMA 8, and 140Wt. Gear oils are all roughly equivalent in oil viscosity - viscosity is precisely what these numbers stand for. ISO VG (International Standards Organization Viscosity Grade), AGMA (American Gear Manufacturers Association), and SAE (Society of Automotive Engineers) are all classifications to represent the different testing methods used to determine oil viscosity, the single-most important factor in determining whether or not a lubricant is applicable for use.
The measurements derived from these testing methods are as follows:
ISO VG 680 oil has a midpoint kinematic viscosity of 680 mm2/s, also referred to as centistokes (cSt). The midpoint is derived by taking the average of both the minimum and maximum kinematic viscosities of these oils, which is taken into account by the AGMA testing method. For AGMA 8 oils, these numbers are 612 cSt and 748 cSt, respectively.