Manual Transmission Fluid Types

by Chuck Ayers
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Transmission fluid provides much-needed lubrication to the complex set of gears and other moving parts in the transmission of a vehicle. Transmission fluid traditionally has been petroleum-based, but with the introduction of synthetic transmission fluid in 1972, synthetic fluid has become increasingly popular because it doesn't lose its viscosity, or coating ability, as quickly. It also is able to transfer heat more evenly for longer periods of time. Both manual and automatic transmissions require transmission fluid of particular viscosity for peak performance, increased power and reduced wear on the moving parts.

Traditional Manual Transmission Fluid

Manual transmissions use a variety of transmission fluid ranging from regular motor oil to heavyweight hypoid gear oil. A few manufacturers even recommend automatic transmission fluid. It all depends on the vehicle, the tolerances, engineering and the type of driving you most commonly do. In manual transmissions, the biggest problem is fluid contamination either due to oxidation or friction between the moving parts that shear minute metal particles into the fluid. This is why it is important to change transmission fluid more frequently in manual transmissions than in automatic transmissions. Over time, the viscosity, or lubricating ability, of the transmission fluid degrades and thins, as well.

Synthetic Manual Transmission Fluid

Synthetic transmission fluids are man-made, produced from refined oils treated with a variety of chemical additives. The first to be developed was from polyalphaolefin (PAO), identified by the American Petroleum Institute (API) as a Group IV oil. Non-PAO synthetics are manufactured from diesters, polyolesters, alklylated napthlenes and alkyklated benzenes and classified by API as Group V oils. Still other synthetic transmission fluids have been developed using feedstock and a catalytic conversion process to produce hydrocracked/hydroisomerized oils, which are classified by API as a Group III transmission fluid or motor oil. The synthetic transmission fluids have been shown to retain their viscosity longer than conventional transmission fluids so they don't have to be changed as frequently. The trade-off is that they cost more than the traditional transmission fluids. Your vehicle manual will indicate which group is appropriate for the transmission in your car.

Traditional vs. Synthetic Transmission Fluids

There is no clear-cut consensus among drivers, mechanics or engineers which type of manual transmission fluid is best. If you drive an Oldsmobile to the grocery store and library twice a week in an urban setting and service the vehicle regularly, the proper weight traditional transmission fluid will serve you well. If you drive a high-performance vehicle under demanding conditions on a frequent basis, synthetic transmission fluid might be a better choice. Either way, the manufacturer's recommended transmission fluid and weight recommendations should be followed. Under either conservative or high-demand conditions, transmission fluids will deteriorate eventually so the most important consideration is regular servicing of the vehicle and replacement of the transmission fluid.

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