Buying guide for Best manual transmission fluid
For those who love driving, a manual transmission is a favorite. Most have more gears than an automatic, so while mastering the gearbox and clutch takes a little practice, the reward is much better control over the performance. Fans of the manual transmission will also tell you that the whole driving experience is more enjoyable.
Manual transmission fluid, also called gear oil, is the lubricant that keeps things cool and running smoothly. Although we tend to take these things for granted, it’s just as vital a part of your drivetrain as any other component. It’s a complex cocktail that can handle tremendous friction and terrific fluctuations in temperature. Inside the transmission there’s a lot of steel, but there can also be comparatively delicate brass guides. The fluid has to deal equally well with the demands of both under pressure.
We’ve been looking at the range of manual transmission fluid available so we can help you choose the right one for your vehicle. Our recommendations cover a selection for different vehicles. The following buying guide expands on the fluid’s composition in more detail and answers some common questions.
Key considerations
Manual transmission types
In Europe and Japan, around 90% of new vehicles have a manual transmission. It’s a very different story in the United States, with only around 15% of vehicles using one. However, percentages were higher in the past, and many classic and collectors’ cars are still used on a regular basis. That being the case, we need to look at the full range of manual transmission fluid types, not just those for today’s cars.
Early manual transmissions were noisy devices with straight-cut gears, technically called a sliding mesh transmission, though the common name is crash gearbox because of the way the cogs crashed together if the speed of the engine and gearbox weren’t carefully matched.
Next came the constant mesh gearbox, and then the synchromesh manual transmission, which is used now. Though synchromesh was first introduced in Cadillacs in the 1920s, vehicles with crash gearboxes — mostly trucks and agricultural vehicles — were still being produced in the 1960s.
Composition
Gear oil can be mineral-based or synthetic, and both are used in a wide variety of vehicles. However, synthetic is much more popular. One disadvantage with mineral oil is that it contains oxygen molecules, and oxygen promotes rust — not something you want in your transmission! Additionally, synthetic manual transmission oil offers better performance over a wider temperature range.
Some people have a misconception that mineral oil is more “natural” and thus more environmentally friendly. However, it’s extracted from crude oil using refining methods that make extensive use of solvents, so it’s not exactly eco-friendly. It’s debatable whether polyethylene glycol or similar products in synthetics are any better, but they’re not a whole lot worse either.
The composition of the oil varies from one manufacturer to the next but will contain the following:
- Oxidation inhibitors to prevent rust
- Demulsifiers to increase lubrication and reduce overheating
- Extreme pressure (EP) additives to improve performance in contact areas.
- Anti-foaming agents
Features
There are two important figures when it comes to manual transmission fluid: performance and viscosity.
Performance: Given as a GL number, this is the fluid’s suitability for different workloads.
Viscosity: Given as an SAE number, this is the thickness of the oil.
Performance
The American Petroleum Institute (API) created a system for automotive gear oils, giving them a gear lubricant (GL) number as a guide to how they work with the various metals in the transmission. The numbers range from GL-1 (lightest) to GL-5 (heaviest). Most modern cars and trucks use GL-4, but it’s worth checking just to be sure.
Viscosity
Viscosity is a more complex subject. At high speeds and with new transmissions in good physical condition, you ideally want low-viscosity (thin) oil. However, under heavy load, you want high-viscosity (thick) oil. The latter is also better at protecting components from wear. So all types of transmission fluid, indeed all types of engine oil, are something of a compromise.
The Society of Automotive Engineers (SAE) developed a system of viscosity ratings that has been adopted by both motor manufacturers and fluid producers and is recognized worldwide. The higher the number, the thicker the oil.
The fluid itself can be divided into two types: monograde and multigrade.
Monograde: These types of oil were widely used until the 1970s and still are in some racing engines. They typically maintain performance for longer than multigrades. However, their big drawback is how they’re affected by temperature. A monograde designated as SAE 90, for example, is great for high ambient temperature applications (up to 212°F), but it doesn’t work well at low temperatures. A different product, SAE W90 (W for winter) is needed if temperatures drop below 0°F.
Multigrade: But nobody wants to be changing their transmission fluid with the seasons. Multigrades are the answer. SAE 75W90, a common rating for manual transmission fluid, has a viscosity of 75 in winter temperatures and a slightly higher 90 rating when it gets hot, and that’s the ideal combination.
The most advanced multigrades have taken this a step further. A decade ago, you almost always bought oil based on the SAE number. Now you’ll see numerous synthetics that satisfy diverse viscosities. One is rated for 75W, 80W, 40, 10W40, and 15W40 — it’s both a gear oil and an engine oil. They can also handle a much wider span of temperatures, for example, from -40°F to 300°F.
It sounds confusing, but as long as it covers the demands of your manual transmission, it’s all good!
Manual transmission fluid prices
In general, when we compile our reports, we like to offer a guide to inexpensive, mid-range, and expensive products. However, most manual transmission fluid costs somewhere between $15 and $22 per quart, so there isn’t a huge variation. We expected to have to pay a premium for a manufacturer’s genuine brand fluid, such as Ford, GMC, or Toyota, but that’s not the case.
The amount of manual transmission fluid your vehicle needs can be anywhere from 6 to 16 quarts, so a difference of five bucks a quart soon adds up. You may be able to save some money buying in bulk, though deals aren’t common. At the end of the day, the most important factor is getting the right manual transmission fluid for your vehicle. If you find several that are suitable, you can then compare prices.
When to change the fluid
Most experts say that, if properly looked after, a manual transmission will last around 120,000 miles, though there are examples of carefully driven cars that have gone twice that distance.
The following symptoms indicate that something could be wrong. Changing the manual transmission fluid may not be the solution, but if you don’t know the last time it was done, then it’s the first thing to try and usually the cheapest. Bear in mind that some of these problems might also be related to the clutch.
- There is a difficulty or inability to shift into gear.
- The vehicle shifts, but the stick feels heavy.
- The vehicle drops down a gear or into neutral unexpectedly.
- You hear whining or grinding noises, particularly when shifting.
Any one of these problems requires immediate attention. The situation will only get worse, and while a new manual transmission is cheaper than an automatic, it’s still likely to cost over $1,500.
FAQ
Q. How often should I change my manual transmission fluid?
A. You should always follow the advice given in your vehicle owner’s manual, but most manufacturers recommend that it be changed every 30,000 to 60,000 miles. If your transmission has to work hard, either through frequent gear changes (typical in very hilly areas) or because you do a lot of towing, you might want to change it more frequently. It does no harm. A regular visual check is a good idea or note if you get any of the problem signs listed above.
Though some modern vehicles have “lifetime” fluids — designed to never need changing — a number of experts still recommend it for vehicles exceeding 100,000 miles. All transmissions wear, and those tiny particles contaminate the fluid, creating more wear. It’s a vicious cycle broken by changing that dirty fluid for clean. Given how infrequently it needs doing, we think it’s worth it.
Q. Can I change the manual transmission fluid myself?
A. If you’re comfortable with basic car maintenance, it should be straightforward. These days there’s a lot of useful guidance online. You’ll need to get your car up on some ramps (never get under it when it’s just on a jack). You’ll also need an oil pan to catch the old fluid. The process can vary depending on the vehicle, but basically, it’s a question of removing the filler plug, then the drain plug, and allowing the transmission to empty, and then reversing the process. When replacing the plugs, it’s a good idea to use new washers, because the old ones might be worn or damaged. Be careful not to overfill when adding the new fluid.
Q. Can I use the same transmission fluid that I use in my automatic vehicle?
A. Possibly, but again, you’ll need to refer to your owner’s manual. Some cars will run quite happily on automatic transmission fluid (ATF). It’s generally a more robust chemical composition because auto transmissions work harder and get hotter. However, never assume that’s the case. You should never mix the two or use ATF to top up manual transmission fluid.
