Viscosity is the oil's resistance to flow or, an oil's speed of flow as measured through a device known as a viscometer. The thicker (higher viscosity) an oil, the slower it will flow. You will see oil viscosity measurement in lube articles stated in kinematic (kv) and absolute (cSt) terms. These are translated into the easier to understand SAE viscosity numbers you see on an oil bottle.
Engine, Automotive Transmission and Gear Oils - An 5W-30 (multigrade) vs an SAE 30 (monograde)
A W on a viscosity rating means that this oil viscosity has been tested at a Colder temperature. The numbers without the W are all tested at 100° C which is considered an approximation of engine operating temperature. In other words, an SAE 30 motor oil is the same viscosity as a 10w-30 or 5W-30 at 100° C. The difference is when the viscosity is tested at a much colder temperature. For example, a 5W-30 motor oil performs like a SAE 5 motor oil at the cold temperature specified, but still has the SAE 30 viscosity at 100° C, which is engine operating temperature. This allows the engine to get quick oil flow when it is started cold verses dry running until lubricant either warms up sufficiently or is finally forced through the engine oil system. The advantages of a low W viscosity number is obvious. The quicker the oil flows cold, the less dry running. Less dry running means much less engine wear.
ISO VISCOSITY CLASSIFICATION SYSTEM
Hydraulic, Industrial Gear, Compressor and Rockdrill Oils
Many petroleum products are graded according to the ISO Viscosity Classification System, approved by the international standards organization (ISO). Each ISO viscosity grade number corresponds to the mid-point of a viscosity range expressed in centistokes (cSt) at 40 deg C. For example, a lubricant with an ISO grade of 32 has a viscosity within the range of 28.8 - 35.2 cSt, the midpoint of which is 32 cSt.
Rule of Thumb: The comparable ISO grade of a given product whose viscosity in SUS at 100 deg F is known can be determined by using the following conversion formula:
SUS @ 100 deg F/5=cSt @ 40 deg C.
COMPARATIVE VISCOSITY CLASSIFICATIONS
Note: Viscosities at various temperatures are related horizontally.
SAE Gear and Crankcase specifications are at 100° C only. Multigrade oil viscosities are not representative at other temperatures.