Manual Transmission and Differential Gear Oils
Most manual transmissions and nearly all differentials require a thicker, high-viscosity oil commonly called "gear oil" or "hypoid gear oil," after the type of gears commonly used in these parts of a car. Some manual transmissions use automatic transmission fluid, however; so check your vehicle's owner's manual or service manual to be sure.
Unlike engine oil, gear oil doesn't have to contend with things like combustion gases; and because most manual transmissions and all differentials are sealed systems, there's little chance of contamination.
What gear oils do have to deal with are relentless and extreme pressure and shear force because of the way hypoid gears slide against each other. That's why all quality gear oils contain EP (Extreme Pressure) additives to protect the gears from wear. Because the oil in the transmission gearbox and in the differential never mix with the oil in the engine, manufacturers can use robust anti-wear additives in gear oil that would be unsuitable for engine oils.
Gear Oil Classifications
The American Petroleum Institute (API) establishes standards for gear oils. At the time of this writing, GL-4 and GL-5 are used in the vast majority of automobile and light truck manual transmissions and differentials. Some heavy-duty vehicles with non-synchronized manual transmissions use MT-1 gear oil, which is a heavier-duty oil.
It's important to note that unlike the "S" and "C" service classifications used for gasoline and diesel engine oils, respectively, gear oils are not cross-compatible, nor are the classifications sequential. In other words, GL-5 oil is not backward-compatible with GL-4; and MT-1 can not be used as a substitute for GL-4 or GL-5, unless the transmission's manufacturer explicity states that it can. Using the wrong gear oil can damage your car's transmission and void its warranty.
Some GL-5 oils claim to also meet the standards of GL-4. I have my doubts. The much higher concentration of EP additives in GL-5 can affect the proper operation of synchro's in transmissions that call for GL-4, and can also corrode yellow metals like brass and copper. When I last replaced the gear oil in one of my own manual-transmission cars that called for GL-4, I used a true GL-4 gear oil made by Redline.
If you simply can't find a true GL-4 oil where you live, and your car's transmission calls for it, then use a dual-rated oil from a well-known, reputable manufacturer. But a true GL-4 is a better choice for transmissions and differentials that call for GL-4.
Gear Oil Viscosity
As with engine oil, in addition to choosing the correct classification, you must also choose gear oil of the correct viscosity. You can find this information in your car's owner's manual or service manual. Typically, vehicle manufacturers will specify a specific viscosity for gear oils rather than a range.
Synthetic Gear Oil
As with automotive oils in general, synthetic gear oils are rapidly replacing conventional, mineral-based gear oils. The advantages of synthetic gear oils include:
- Better lubricity to protect the transmission's or differential's moving parts.
- Longer service life.
- Much easier shifting, especially in extremely cold weather.
- Better inherent thermal viscosity stability reduces or eliminates the need for long-chain polymers as viscosity improvers, resulting in less degradation due to shear.
- Quieter, cooler operation.
- In most cases, a slight improvement in fuel economy.
By way of a personal anecdote, the last time I replaced the conventional gear oil in one of my manual-transmission cars with synthetic, I found myself shifting late (that is, letting the engine rev higher before I shifted). That was because unconsciously, I'd been using the transmission whining as a audible tachometer. The synthetic completely eliminated the whining, causing me to shift late for the first hundred miles or so.
Changing Manual Transmission or Differential Oil
Gear oils tend to last a very long time, but not forever. Like everything else, gear oils eventually wear out. Usually it's the additives, rather than the base oil, that break down.
If your vehicle's owner's manual or service manual specifies a replacement interval for the manual transmission oil or differential oil, you should follow it. Since the advent of synthetic gear oils, however, many manufacturers consider these oils to be "lifetime" and don't specify a change interval. Bear in mind, however, that "lifetime" usually means the length of the warranty, not forever. If you still own your car when the warranty runs out, you may want to change the manual transmission oil and differential oil at that time.
If you choose to change the manual transmission and differential oil in your car yourself, make sure to observe good safety practices. Use jack stands if you have to get under the car, wheel chocks to keep the car from rolling, chemical-resistant gloves, and eye protection.
The exact procedure for changing the oil in either the manual transmission gearbox or the differential varies by car make and model. Consult your vehicle's service manual for detailed instructions. Typically, however, it's an easy job.
Most manual transmission and differential gearboxes have two threaded holes with bolts in them, one on the bottom of the gearbox and one near the top. The hole on the bottom is the drain hole, and the one near the top is the fill hole. The typical gear oil change procedure is:
- Drive the car around a bit to warm the oil.
- Park the car, raise it as little as possible, support it on jack stands, and chock the wheels. You really want the car to be as level as possible; so if you can reach the fill and drain holes without jacking up the car, great.
- It might be possible to do the job mainly from under the hood on some vehicles if you remove things that are in the way. For example, maybe you can reach the fill hole if you remove the battery. Think it through and do it from the top, if possible.
- Always, always, always remove the upper bolt (the one in the fill hole) first to make sure it comes out. You don't want to drain the oil and then find that you can't refill the gearbox because the top bolt won't budge or because you snapped off the head trying to remove it.
- Remove the bottom bolt and drain the oil into an oil drain pan. If you had to jack up the car, you may want to lower it at this point to let the oil drain fully.
- Replace the bottom bolt and torque it to the manufacturer-specified torque using a torque wrench.
- Slowly fill the gearbox through the top hole using an oil filler spout of a style that makes the most sense given the specifics of your car. Typically you fill the gearbox until the oil starts coming out the fill hole.
- Tighten the bolt in the fill hole finger-tight and shift through the gears with the clutch depressed. You shouldn't need to start the engine.
- Remove the bolt and see if it takes any more oil. When it's full, tighten the bolt to the manufacturer-specified torque.
- Test-drive the car.
Again, these steps are general in nature. Consult your vehicle's service manual for more complete and specific instructions.