- This is always going to be the safest bet, especially these days when manufacturers routinely build transmissions with a certain type of oil in mind. This is especially true of automatic transmission fluid (ATF), which can contain dozens of different friction modifiers, specialized anti-wear additives, detergents and anti-foaming agents. Modern transmissions rely on a given friction coefficient and viscosity just to function, so you play with these factors at your peril. Always consult the oil manufacturer to determine if the lubricant is suitable for your application.
Racing Factors (Manual Transmissions)
- Manual transmissions require a very slippery fluid to reduce friction, a stable fluid that will maintain its viscosity (thickness) under high-heat conditions and a molecularly strong fluid that will resist breakdown under the extreme shearing and pressure forces exerted between the gears.
Racing Factors (Automatic Transmissions)
- Heat is the automatic transmission fluid's greatest enemy, which is why most racing fluids are formulated to control heat and resist thermal breakdown above all else. Manufacturers accomplish this task partially by making the fluid as slippery as possible, often by incorporating suspended metals such as molybdenum and elements like sulfur. Like manual transmissions, automatic transmission also require a formula that can resist shearing and pressure-induced breakdown.
Gear Oil Types
- Automotive gear oils come in six basic categories, ranging from GL-1 to GL-6. The GL-1 and GL-2 are only suitable for fairly small engines powering lightweight vehicles. GL-3 is generally acceptable for most automobiles and may contain anti-wear and anti-friction additives. GL-4 is the strongest fluid available that doesn't use sulfur, which can damage brass and bronze components. GL-5 oils are suitable for all but the most extreme circumstances (such as tractor-trailer racing and Top Fuel), but may or may not contain sulfur. GL-6 is for the heaviest of heavy-duty applications. A GL-4 will work under most circumstances, but may not be heavy-duty enough to resist shearing between the ring and pinion gears present in many racing transaxles. Again, consult the transmission and oil manufacturer to check for compatibility.
- Many modern, front-wheel-drive vehicles use the same oil to lubricate the transmission and final-drive gearing. This goes for manual and automatic transaxles alike, but don't assume that just because your car uses a manual transmission that it requires a traditional manual gear oil. Some manufacturers prefer to use a standard transmission fluid in their manual boxes instead of gear oil, especially those that use computer-controlled automated manuals.
Sytnthetic vs. Mineral
- A synthetic oil will almost always outperform a mineral oil in terms of resistance to thermal breakdown, viscosity retention, shear/pressure resistance, low-temperature performance and lubricity (slipperiness). However, synthetics tend to be a bit more reactive in the presence of water, meaning that they don't protect as well against corrosion and may actually absorb water vapor. Synthetic oils may not be compatible with certain mineral oils, some plastics derived from petroleum and certain metals. However, the average automatic or manual transmission will probably live a far longer and cooler life with a synthetic than without one.