AMSOIL Severe Gear Provides

Unsurpassed Shear Stability

Model-year 2007 turbo diesel pickup trucks,

V-10 gasoline pickups and sport utility vehicles

(SUVs), and high-horsepower V-8 trucks

have more towing and payload capacities than

in previous years, yet their differentials have not

changed.

There has been a 34% increase in

engine horsepower over the last decade, while axle gear sizes have

remained constant, sump capacities have been lowered and drain

intervals extended. In the light truck segment there has been a 93%

horsepower increase since 1981. In vehicles such as a fifthwheel

equipped Ford F-350 Super Duty, towing capacities have

reached a high of 19,200 lbs. And testing shows that in new axle

applications simulating trailer towing at 88 km/h (55 mph) at a

3.5% grade temperatures can reach as high as 188°C (370°F).

Stress on differentials has also increased in limousines, conversion

vans, and trucks and cars with modified, high-performance

engines. More power, more towing capacity and higher hauling

limits greatly increase the stress that causes heat and wear.

Most vehicles operate under severe service as defined by

vehicle manufacturers, but the majority of vehicle owners are

unaware of this. Severe service applications include towing,

hauling, plowing, off-road use, frequent stop-and-go driving,

steep-hill driving and temperature extremes. Severe service

applications are on the rise. For example, more than 90 percent

of Ford Super Duty pickups are used for towing. Severe service

increases the need for better gear lubrication.

AMSOIL Severe Gear Gear Lubes provide superior performance

and protection. Their well-balanced synthetic formulation

reduces friction and wear and improves efficiency. In fact,

AMSOIL recently completed a white paper entitled A Study of

Automotive Gear Lubricants (G2457) that proves AMSOIL

Severe Gear Synthetic 75W-90 Gear Lube’s superiority. This issue

of Service Line examines the results of one of the tests from the

white paper, the KRL 20-Hour Shear Stability Test.

KRL Shear Stability Test

Similar to 5W-30 automotive engine oils, 75W-90 gear lubricants

are defined as multi-viscosity. This means the gear lubricant

has enough viscosity to protect against wear at high temperatures,

as well as good flow properties at cold temperatures. Many gear

lubes cannot fulfill both requirements without the use of VI

improver additives. VI additives keep lubricants from becoming

too thick to flow in cold temperatures and too thin to protect in

high temperatures. VI additives have many uses. If used improperly

in gear lubricants, however, they can break down and lose viscosity

through a process called shearing.

Because of this, the SAEincorporated the CEC L-45-A-99 (KRL) 20-Hour Shear Test as

a requirement for all automotive gear lubes. This specification

requires that gear lubes not shear down and fall below the minimum

viscosity for that grade. Testing showed that Lucas 75/90

Synthetic, at 22.35 centistokes (cSt), and Royal Purple Max-Gear

75W-90, at 19.32 cSt, both exceed the maximum 18.49 cSt initial

viscosity, failing the SAE J306 requirements for SAE 90 gear

lubes. All other gear lubricants were within the required high-temperature

viscosity range prior to the KRL Shear Stability Test.

Viscosity measurements following the KRL test revealed that

seven gear lubes sheared down below minimum viscosity requirements,

failing the shear stability requirements of the SAE J306.

The two gear lubes with the largest viscosity loss, as reflected in

the following graph, were Royal Purple, losing 40.6% of its viscosity,

and Torco SGO Synthetic, losing 35.2% of its viscosity.

Royal Purple was the only gear lube to fail both the initial viscosity

requirements and the shear stability requirements. It started

out too thick and ended up too thin. Torco SGO Synthetic finished

the shear stability test as the thinnest of all the oils at 9.97 cSt,

far below the minimum 13.5 cSt requirement. Lucas 75/90

Synthetic, with an initial viscosity that exceeded the maximum

requirements by 20.8%, passed the shear stability test, but lost

34.5% of its viscosity, the third largest loss of viscosity. Both

OEM gear lubes, GM and Mopar, failed the minimum viscosity

requirements after the shear test.

Of all the gear lubes tested, half

did not meet the SAE J306 shear stability requirements.

AMSOIL Severe Gear 75W-90 was in the proper initial viscosity

range and retained the highest viscosity after the shear test

with a viscosity of 16.03 cSt – the mid-point of the SAE 90 viscosity

grade.