Understanding Multi-Viscosity Motor Oils-best auto zone

I've lost track of how many times a customer has picked up...say a bottle of 5W/30 motor oil in my office, looked at the numbers on the bottle and asked me "so this oil is a 5 weight 30"?

Vehicle owners usually know that their car requires a certain viscosity and grade of motor oil as per their owners manual but more often than not don't know what these numbers mean.

The fact is that very few consumers understand today's multi-viscosity motor oil ratings or the nature of how these modern motor oil work as relates to the oils viscosity.

Viscosity is defined as a fluids resistance to flow at a specific temperature.

All multi viscosity motor oils will have two sets of numbers. The top (larger) number indicates the oils viscosity. The most common measurement of oil viscosity is called kinematic viscosity, which measures how fast the oil will flow down through a glass tube heated to 100 degrees centigrade (212F) when pulled through by gravity.

Kinematic viscosity is measured in units called centistokes (cSt) For a motor oil to meet the 30-weight-viscosity range it must fall between 9.3 and 12.5 centistokes.

The bottom (smaller) number with the "W" indicates how the oil flows when it's cold. This number indicates the motor oils winter viscosity rating.

So what 's the difference between say 5W/30 and 10W/30 motor oil?

First they are both 30 weight motor oils and there is no difference at all when either is in an engine at operating temperature. So both a 5W/30 and a 10W/30 have the same resistance to flow, the oils viscosity, when the engine is running and warmed up to operating temperature.

At cold temperatures though, the use of additives and or polymer technology allow the motor oil to flow more readily, i.e. a 5W/30 motor oil will flow more easily at low temperatures than a 10W/30 and a 0W/30 will flow better still at low temperatures making them more suited to winter use then the 10W/30

This is why the old practice of changing straight grade oils to a heavier viscosity in the summer and a lighter viscosity in the winter is no longer necessary.

So if 0W/30, 5W/30 and 10W/30 are all 30 weight motor oils at operating temperature (kinematic viscosity of 9.3 to 12.5 centistokes at 100 C.) how is the lower winter rating number determined?

The winter or cold weather rating uses two measurements called Brookfield cranking viscosity and Brookfield pumping viscosity, both measured in units knows as centipoise. (cP) These tests measure the oils resistance to flow at increasingly lower temperatures for each winter rating.

For a 30-weight oil to receive a 10W winter rating its Brookfield cranking viscosity must not exceed 7000 cP at -25C and its Brookfield pumping viscosity must be less then 60,000 cP at -30C.

For a 30-weight oil to receive a 5W winter rating its Brookfield cranking viscosity must not exceed 6,600 cP at -30C and its Brookfield pumping viscosity must be less then 60,000 cP at -35C. Obviously 0W/30 motor oil would have even lower numbers at even lower temperatures.

I will note again that all three of these 30-wt oils would be in the same kinematic viscosity range measured at 100C (30-wt-viscosity range) it is only the "W", winter rating that would be different.

Aside from kinematic and Brookfield viscosity tests each motor oil weight classification also has its own high temperature/ high shear (HT/HS) requirements that the oil must meet.

This HT/HS test simulates an engine running under normal load conditions and measures in centistokes how well the oil maintains its viscosity over a given amount of time at a temperature of 150C (302F)

Motor oil must be at a minimum of 2.9 cSt to meet the HT/HS requirements for the 30-wt range.

It should be noted that when talking about fully synthetic motor oils the same tests apply but that the performance capabilities of the oil in both hot and cold temperature extremes moves out in each direction of the scale.

Petroleum oil will thin out far more at high temperatures than synthetic oil will. The synthetic oil has the ability to handle far high temperatures with out volatilizing and becoming thicker or oxidizing, breaking down and shearing (thinning out to an unacceptable degree).

Because synthetic oil does not contain paraffin (wax) and other impurities like petroleum oil and because of its uniform molecular structure, synthetic oil can flow and operate in sub-zero temperatures that would cause petroleum oil to solidify making it ideal for use in demanding high and low temperature service.

Amsoil Inc. introduced the first full synthetic motor oil to meet American Petroleum Institute service requirements in 1972. Today Amsoil is considered the world leader in synthetic lubrication.
By Larry Crider

Larry Crider is a certified lubrication specialist and a 20+ year veteran distributor for Amsoil inc., makers of high quality synthetic lubricants for nearly any application. To contact Larry or to get a free Amsoil products catalog by mail go to http://www.sowoil.com