In this issue I have elected to discuss
Automotive Gear oils & lubrication for our
vehicles. Gear oils are a very important issue
when we are dealing with increased
horsepower vehicles, and coupling the power
to the ground.
To enter this discussion it is important to start
with some basic fundamentals about
lubrication. Lubrication of mechanical
components is generally accomplished by
maintaining two primary types of lubrication,
Hydrodynamic Lubrication & Boundary
or “Full Fluid
Film” lubrication is accomplished from
mechanically engineering oil pressure, or by
using high viscosity fluids to maintain a fluid
wedge to float the bearing surfaces upon. In
this type of lubrication, the parts “surf” upon a
cushion of oil and do not make excessive
contact with the other surface, and the
surfaces in theory do not come into severe
contact with one another.
is accomplished by
having a thin, strong film, which provides
chemical friction reduction & load wear
protection. This method of providing lubrication
generally relies upon the chemical structure of
the lubricant to maintain a very thin chemical
barrier to prevent wear and damage to the load
In gear lubrication, both methods of
maintaining lubrication are important
considerations. The viscosity
of the lubricant
must be correct to provide a fluid cushion to
protect the surfaces from very high shock loads, and the chemistry
of the product must
be correct to protect the surfaces during
contact & sliding motion.
Automotive Gear oil chemistry differs
considerably from motor oil chemistry in the
types of anti-wear additives that are used in the
Motor oils rely upon a Zinc-Calcium anti-wear
structure, and Gear oils generally rely upon a
Sulfur-Phosphorus anti-wear structure.
(Considerable other differences exist in each
of the types of fluids as well).
Temperature and pressure activate the
chemical EP (Extreme Pressure) components
in gear oils. This chemical reaction provides
slippage & protection to the gear face. Gear
lubricants have a very distinctive odor
associated with them, and this is generally due
to their additive structure.
The performance and application of Gear oils
are identified by their viscosity expressed in an
SAE viscosity grade, and by an API (American
Petroleum Institute) GL specification number.
The API - GL specification number is an
indication of the intended gear type, load, and
material for the application. The numbering
scale progresses upward for load carrying
ability with GL 3&4 products being primarily
intended for low load automotive gearbox &
manual transmissions, and Hypoid / GL 5 & 6*
rated products being intended for higher load
applications such as in differentials and rearends.
*The API Severe duty GL6 standard has been
removed from warranty service specifications
for new light duty passenger cars, but some oil
manufacturers maintain the more severe
capability oil performance specification for
For the enthusiast wanting maximum
protection, it is my opinion that the additional
protection that is offered by a “GL-6” rated
product is a worthwhile investment.
Many gear oils maintain chemistry to reduce
chemical attack of soft metals, and when the
consumer selects a product, they should
always be aware of what type & specification of
lubricant the equipment manufacturer
recommends for the application.
One important thing to point out with gear
lubricants is that they are not classified on the
same viscosity scale as crankcase motor oils,
and their viscosity relationship (not Load
Carrying Ability or application read-across) may be approximately summarized as follows,
75w90 gear oil = 10w40 motor oil, 85w gear oil
= SAE 30 motor oil, 90w gear oil = SAE 40
The chart gives an accurate visual relationship
of viscosities of different oils that are typically
used for gear lubrication.
In the high performance & specialty
applications the consumer needs to take into
consideration how they will be using their
equipment and what other variables exist.
example, in some applications where a rear
end or transmission has been custom built or
modified, the rebuilding engineer may have set
the gear contact area & clearances for street,
drag racing, or road racing. With or without
limited slip, or with numerous other variables.
The re-builder may have set the clearances for
a light viscosity product such as a 75w90 for
road racing, where the loads are constant and
the speeds are high. Or have set the
clearances to withstand severe shock loads
such as in drag racing, where a higher
viscosity product such as a 90w140 or higher
viscosity may be necessary.
In performance street applications the
equipment manufacturer or re-builder may
specify a mid-viscosity product such as a 90w
to provide gear protection and to limit gear
noise. And in some cases an additional
additive may be necessary for different
applications with limited slip or posi-traction.
Tips For Sport Enthusiasts
Synthetic gear oils will generally evacuate
foam faster than petroleum oils, and will take
more temperature prior to degrading. Synthetic
products usually have less fluid drag and have
a more positive feel for shifting.
Petroleum oils will usually be the factory-fill
oils, particularly in old vehicles. These oils may
have degraded with use & time and should be
checked. Top up with a similar viscosity & type
of product if the system is low. In the case of
very old vehicles, synthetic oils may not be the
ideal products because of worn seals & looser
tolerances may promote leakage.
Change out gear & differential oil occasionally
because as gears wear, they generally shed
metal, and it is good to keep “random” metal
flakes out of the gears.
• Always use the correct “API GL” service
specification for the gear type and application!
• Never use Gear Oils for engine
Gears “stir” air into the fluid. In the event that
the system gets contaminated with water,
change the fluid! In some instances when
water has contaminated the gear lubrication system, multiple oil changes may be necessary
to get rid of the water traces that promote
Water turns to steam, and as the lubricant is
removed from the gear surface it promotes
wear and no fluid cushion is available for shock
load protection. Foam & steam pressurizes the
gear case, and may blow the lubricant past a
seal, and when water gets in, it usually brings
In some instances gears may not get much
lubrication because of designs that exist. In
cases such as these, an oil that webs may be
benificial, as the oil will be transfured to the
gears at the top of the gear case. This is
usually beneficial in low speed gear cases, and
for gears that do not experience extreme loads,
temperatures or speeds.
This chart shows horsepower changes related
exclusively to the paracitic losses in the
differential from the oil. This data was
measured on a Dynojet 248C Chassis
Dynamometer. (Data courtesey of Dr. Joseph Donnelly.)
In many cases it is difficult to know which gear
oil is the best for your application. The best
way to select a gear oil is to understand the
specifications and to know what type of service
requirement your application requires.
For Your Information…. If your gear application requires extra
protection, and minimal paracitic losses related
to the gear oil, Torco manufacture’s a variety of
Gear oils. The gear oils that Torco
manufactures have an easy to understand
Synthetic ATF: This product is intended for
heavy duty high heat applications where the
fluid must remain ultra consistent.
MTF: Manual Transmission Fluid. These
products are available in petroleum,
synthethetic blend, and full synthetic
formulations. A variety of viscosity grades are
available, with formulations for specific
Racing Gear Oil GL-6: These petroleum
based oils are available in a variety of viscosity grades ranging from 80w90 to 85w140. Also a
very high viscosity top fuel version is available.
SGO - Synthetic Gear Oil GL-6: These high
performance gear oils are available to
withstand severe heat applications, and have
minimal fluid drag chacteristics.
RTF – Racing Transaxle Fluid: This racing
transmission fluid is designed for applications
where minimal fluid resistance is required, and
the gears run under constant high speed
For more complete information on the gear oils
that are available, please refur to the product
data sheets related to the specific Torco gear