ISO VG for Engine Oils

ISO Viscosity Grade	Kinematic Viscosity (cS @ 40 deg C)
	Minimum	Maximum
ISO VG 2	1.98	2.42
ISO VG 3	2.88	3.52
ISO VG 5	4.14	5.06
ISO VG 7	6.12	7.48
ISO VG 10	9	11.0
ISO VG 15	13.5	16.5
ISO VG 22	19.8	24.2
ISO VG 32	28.8	35.2
ISO VG 46	41.4	50.6
ISO VG 68	61.2	74.8
ISO VG 100	90	110
ISO VG 150	135	165
ISO VG 220	198	242
ISO VG 320	288	352
ISO VG 460	414	506
ISO VG 680	612	748
ISO VG 1000	900	1100
ISO VG 1500	1350	1650

There are two basic methods of specifying the viscosity of fluids: absolute and kinematic viscosity. Viscosity index is an arbitrary measure of a fluid resistance to viscosity change with temperature changes. Thus, viscosity is affected by temperature changes. As temperature increases, the viscosity of a fluid decreases. A fluid that has a relatively stable viscosity at temperature extremes has a high viscosity index. A fluid that is very thick  while cold and  very thin while hot has a low viscosity index.

Factors Influencing the Selection of a Fluid
The selection of a hydraulic fluid for a given system is governed by the following factors:

• Operating pressure of the system.
• Operating temperature of the system and its variation.
• Material of the system and its compatibility with oil used.
• Speed of operation.
• Availability of replacement fluid.
• Cost of transmission lines.
• Contamination possibilities.
• Environmental condition (fire proneness, extreme atmosphere like in mining, etc.).
  Lubricity
• Safety to operator.
• Expected service life.