The environment within a gearbox is extreme. Gears either come into and out of mesh, or they are in constant contact, but the only thing preventing gearbox damage is gear oil. Not all gear oils work in every situation, though. Read on to find the best gear lubricant to use.
The Best Gear Lubricants
Brand & Product Name
Why Gear Oil?
Whether in a manual transmission, transfer case or differential, all automobiles use gear oil of one form or another. Depending on the configuration of the gears, the oil is required to remedy certain conditions. Like motor oil, gear oil has specific tasks. (This article on Popular Mechanics’ website describes differentials and explains how to service them.)
The best gear lubricant will prevent the metal-to-metal contact that causes friction and wears gears down, and it removes heat from around gears. The gears sit in a sump and carry oil with them as they turn. Gear oil’s high viscosity helps it coat the gears and lubricate the bearings.
Synthetic gear oil offers several advantages over conventional oil that consumers would be wise to implement. First, it flows better in the cold and retains its initial viscosity in the heat better than dinosaur oil. Second, synthetics retain their properties longer, extending drain intervals.
Gear Oil Additives
Gear oil must overcome the high pressure in some differentials. Extreme pressure (EP) additives help gear oil retain its lubricating qualities in such conditions. (This article explains EP additives in more detail than there is space for here.) Sulfur was long the EP additive of choice, but its corrosive properties has led some manufacturers to find less volatile substances.
Sulfurized synthetic additives are now less reactive, and phosphorus-based additives contribute lubricity, as well as EP properties to gear oils. Other additives found in gear oils include metal deactivators to prevent corrosion, anti-rust agents, friction modifiers and dispersants.
API GL Ratings
The American Petroleum Institute defines four performance categories of gear oil. The API GL-1 designation is for manual transmission oils without friction-modifying, EP or anti-wear additives. GL-4 lubricants have these additives, and are used in differentials that operate in mild conditions.
The API GL-5 category designates oils utilizing high amounts of EP and anti-wear additives. It is used in the most severe conditions, such as hypoid gear differentials. API MT-1 oil is for manual transmissions that lack synchronizers, thus requiring more thermal stability than GL-5 oils.
The Best Gear Lubricants
1. Mobil 1 Synthetic Gear Lube LS
Thanks in part to synthetic base oils, Mobil 1 Synthetic Gear Lube LS is stable across a wide range of temperatures. It remains fluid to -50 degrees Celsius, yet it retains its viscosity even in hot-running, high-performance differentials and rear axles, including limited-slip differentials.
Mobil 1’s additive package includes sulfur and phosphorus, but does not corrode yellow metals like brass bearing cages. It is suitable for all gear oil applications requiring an API GL-5 rating. It extends drain intervals while reducing wear thanks to its phenomenal shear stability.
2. Red Line MT-90
Manual transmissions require a gear oil with reduced slipperiness to improve synchronizer function. Red Line MT-90 is an API GL-4 synthetic gear lube that protects synchronizers and gears from wear. Its additive package also enables easier shifting in cold temperatures.
Like all GL-4 lubricants, MT-90 lacks the sulfur additives that can corrode and damage brass and copper manual transmission components. Its base stock is ester-based, so it is shear-stable in most operating temperatures. MT-90 is suitable for GM, Ford, VW, Audi and many other applications.
3. Royal Purple Max Gear
Royal Purple enjoys a reputation for high performance and long drain intervals, and its Max Gear lubricant lives up to the hype. Max Gear is fully synthetic, so it flows in cold temps and retains its viscosity in hot-running, heavy-duty hypoid differentials and other GL-5 applications.
Unlike many other gear lubricants, Max Gear is also suitable for GL-4 applications like manual transmissions and front-wheel-drive transaxles. Royal Purple manages to satisfy both ratings with its proprietary Synerlec additive package, which does not harm soft metals in synchronizers.
4. Lucas Synthetic Gear Oil
Lucas lubricants have a reputation as simply being a quick fix, delaying costly repairs. While true, Lucas Trans and Diff Lube prevents many such issues from ever occurring. This GL-5 rated lube is extremely slippery, reducing wear and friction in hypoid and limited-slip differentials.
As a fully synthetic lubricant, Lucas Gear Oil’s viscosity is stable in extremely cold or hot conditions. It can smooth operation in aging gearboxes as well, but its high sulfur content suggests it may damage brass bearing cages or other yellow metals in manual transmissions.
5. Valvoline SynPower Full Synthetic Gear Oil
Valvoline’s SynPower is fully synthetic gear oil that meets API GL-5, MT-1 an MIL-PRF-2105E specifications. Its literature suggests it is also GL-4 applicable, but it contains sulfur additives that will corrode synchronizers. It shines in limited-slip differentials, though.
SynPower uses synthetic base stocks, so it is stable in extreme temperatures. Its additive package ensures it remains between gear teeth even in extreme pressures and high heat, and it contains effective anticorrosion additives as well. It is likely the best gear lubricant for those looking to go full synthetic yet stay on budget.
6. Castrol Syntrax Limited Slip
Syntrax is considered a fully synthetic gear oil, though Castrol uses Group III base oils in its production. It offers thermal stability and shear stability on par with most of the best gear lubricants, though. It also boasts extended drain intervals over conventional gear oils.
- Outperforms similarly priced conventional gear oils
- Saves money over other full synthetics
- API GL-5 and MT-1 approved
- Improves fuel mileage over old, dirty gear oil
- Formulated for extreme-pressure environments
- Made of highly refined conventional oil
- Costs more than Valvoline with no real benefit
It can be difficult to isolate a gear oil that benefits a particular vehicle the most. If the correct viscosity and API rating are used, any synthetic gear lube will do the job. The best gear lubricant is the one specifically formulated for a particular differential or transmission. Synthetic gear oils extend drain intervals and are more thermally stable across a wide range of temperatures as well. Conventional oils cost less than synthetics, but as this article points out, the savings are only worth it if the cost of mechanical failure is negligible. Differentials and transmissions hardly fit that description.