Motor oil , Gear lube & Auto. Trans. Fluid
What’s the Difference?
What are the differences between motor oil, gear lube and automatic transmission fluid? Why do those differences matter?
Lubricants, such as motor oil, automatic transmission fluid and gear lube, contain lubricating basestocks and additives that provide performance benefits the basestocks cannot provide.
Motor Oil - Motor oil additives include dispersants and detergent/alkalinity additives to keep contaminants in suspension until they are carried to the engine filter where they are removed and to neutralize the acids created by combustion. Motor oils also contain emulsifiers to suspend water, a combustion byproduct, in the motor oil. Suspended water evaporates in high temperature areas of the engine.
Automatic Transmission Fluid
Of the three lubricants considered here, automatic transmission fluid(ATF) has the highest viscosity index.
While all three automotive lubes may be called upon to function in temperatures ranging from extremely low to extremely high, ATF alone functions as a hydraulic fluid. To ensure consistent shifting, the automatic transmission’s hydraulic cylinder must move at roughly the same rate whenever it is engaged. The high viscosity index of ATF ensures that fluid viscosity remains relatively constant despite temperature fluctuations.
Gear Lube- Gear lubes contain extreme pressure additives to protect gear surfaces. Gears often operate without a full lubricating film separating them and some gears contact their mating gear surface along a line or single point of contact Extreme pressure additives chemically interact with gear surfaces and form a “shield” that protects the gear surfaces from contacting and damaging one another. Gear motion continually shears the shield from the gear surfaces and it is continually replaced by the chemical interaction of the lubricant and gear surface.
Gear lubes also contain tackifiers, which help the lubricant cling to gear surfaces and transfer from one gear to the next.
ATF and gear lube contain demulsifiers, which helps water separate from the lubricant. The applications in which these lubes are used are not exposed to combustion, so water build up is uncommon.
Cross-applications
Engine- Because automatic transmissions, manual transmissions and differentials are not exposed to combustion processes, ATF and gear lube contain no dispersants or detergent/alkalinity additives. These lubes would give an engine no protection from the corrosive effects of combustion acids, nor would they suspend contaminants for delivery to the filter, where they are removed.
The lack of emulsifiers would allow a buildup of water in the lubrication system.
The chemistry that allows gear lube extreme pressure additives to form a protective shield on gear surfaces may corrode some engine bearings.
Automatic transmission- Motor oil dispersants and detergents would promote wear in an automatic transmission by keeping contaminants suspended and circulating. The automatic transmission filter is small and oriented in a way that allows contaminants to drop off when the vehicle is shut down. With contaminants suspended by the dispersants and detergents, the filter would plug quickly and inhibit fluid flow, potentially leading to transmission failure.
As they may in the engine, gear lube extreme pressure additives may damage sensitive metals in the automatic transmission.
Finally the lower viscosity indices of motor oils and gear lubes would impair shift consistency.
Gears- With no extreme pressure additives, motor oil and ATF would allow excessive wear in the gears, such as differentials and some manual transmissions, that require gear lube.
With no tackifiers, motor oil and ATF would not cling sufficiently to protect gear surfaces.
Finally, motor oil emulsifiers may encourage foaming and rust.
