The International Standards Organization assigns industrial lubricants numbers based on their Viscosity Grade.
ISO VG 15 are commonly employed in hydraulic systems and many other industrial applications operating in cold weather and also in (some) hydraulics applications requiring a MIL-H-5606 lubricant. ISO VG 15 oils may also be used in certain spindle and air tool applications.
ISO VG 15 oil has a midpoint kinematic viscosity of 15 mm2/s, or centistokes (cSt). These numbers measure an oil's resistance to flow (i.e. viscosity).