What Fluids Are In A Car?

Fluids play an essential role in every car. Oils help lubricate moving parts and absorb heat. Hydraulic fluids keep things like your brakes, steering, and clutch working. Other fluids keep your AC running, your windshield clean, and your engine from overheating.

With all those fluids to keep track of, it can be hard to remember what they do and when to replace them. We put together a list of the most important fluids in a car to help you do just that.

Car Fluids List

Below is a list of fluids in a car, with descriptions and maintenance cycles. This table covers the most common fluids used in cars today. Keep in mind that every vehicle will have different fluids and maintenance recommendations. Remember to always read your owner’s manual for specific guidance.

Fuels

We’ll get the obvious one out of the way first. Unless your vehicle is fully electric, it will need a fuel of some kind. This is most likely gasoline or diesel, but could be something like hydrogen or natural gas.

The low fuel light is one we've probably all seen before. Hopefully you usually fill up before it turns on.

Standard gasoline fuels (petrol) are divided by their octane level and ethanol level. Different regions have different octane measurement systems, as well as different ethanol uses. Diesel has different varieties as well, including off-road diesel, which isn't legal for on-road use.

Fuel levels are displayed on your fuel gauge. Provided that your gauge is accurate, you’ll simply want to check it after starting the car and before driving.

Gas can go bad, so you shouldn’t leave it in your car for months on end if you’re storing it. This is something to keep in mind if you’re storing a car for winter. Fuel stabilizers can help but have specific requirements that you must read carefully.

Oil and Lubricants

Where two moving parts come into contact, oil is necessary as a lubricant. It allows for smooth, precise movement while absorbing heat.

Motor Oil

Besides gasoline, motor oil (also called engine oil) is probably the best-known liquid in a vehicle. Without motor oil, your car's engine wouldn't last long. Not only would it grind itself into a pulp, but it would also get covered in all sorts of deposits. Plus, oil absorbs heat to help keep your engine from overheating. Different types of oil need to be changed at different intervals, so be sure to refer to your car's manual.

Is Synthetic Oil Better than Regular Oil?

Synthetic oils generally last longer than conventional oils. Performance vehicles will often require, or at least recommended, full-synthetic oil. Synthetic oil won’t hurt a non-performance vehicle, but it costs more and might not be necessary.

Semi-synthetic (blended) oils are cheaper and offer a good compromise for many. Oils that are partially or fully synthetic offer longer-lasting protection. They also tend to do better with short trips where regular oil won’t have the chance to fully warm up. Finally, you’ll see better results in very hot or very cold weather.

How Do You Read Oil Numbers?

The numbers you see on oil containers refer to the oil’s viscosity. Your engine may require a certain type, which will be listed in your owner’s manual. Typical oil viscosity ranges from 0W-20 to 20W-50.

A lower number means thinner oil. The number with the “W” indicates the oil’s viscosity at lower temperatures, while the second number indicates the viscosity after the oil has heated up. The W stands for “winter,” indicating that the oil can be used in cold temperatures (as most modern oils can).

How Often Should You Check Your Oil?

Oil is one of the most important fluids to check. It’s a good idea to check it every few weeks and before you go on a long car trip. It’s likely that your car’s computer will give you a recommended oil change warning. Keep in mind that sometimes this is based on a complex algorithm to calculate oil quality. But it’s most often based on mileage or time.

New motor oil will be amber and look fresh. Oil that needs to be change can be watery or sludgy, often dark brown or black in color.

Checking your oil level is as simple as removing the dipstick, wiping it off, re-inserting it, and pulling it out again. Ensure that the oil line is between the high and low marks. You should also check to make sure the oil is a gold color. New oil will be gold-yellow, and it will naturally get more orange over time. However, a dark brown oil that is sludgy or watery should be replaced. A milky-colored oil indicates that there may be a coolant leak.

Should I Check the Oil When the Engine Is Hot or Cold?

Despite popular belief, you don't need to wait for your engine to be warm to check the oil. In fact, you should avoid checking the oil when the engine is still hot. Wait at least 5-10 minutes after shutting the engine off to be safe. For many cars, you can simply check the oil before you start driving. Check your owner’s manual to see what’s recommended for your vehicle.

How Often Should You Change Your Oil?

It’s important to follow the recommendation of the manufacturer. While older vehicles often had a recommended oil lifetime of 3,000-5,000 miles, modern cars can usually go 7,500-10,000 miles between changes. Sometimes even more. This is especially true if you’re using synthetic oil.

The oil pressure light means a problem with your engine oil. As with any red dashboard light, you should immediately pull over when safe.

Cars with direct-injection engines, especially those that are turbocharged or supercharged, might need their oil changed more often. Doing so can help prevent carbon build-up.

Don’t forget to change your oil filter when you change your oil.

Transmission Fluid

Transmission fluid is similar in function to motor oil, except it doesn't need as much attention. Transmission fluid provides lubrication that keeps the gears in your gearbox from eating each other up. Like engine oil, it also helps keep your transmission from overheating by absorbing heat.

How Often Should You Change Your Transmission Fluid?

Manual transmissions generally need new transmission fluid every 30,000-60,000 miles, and automatic transmissions need changing from 30,000 miles to 100,000. CVT transmissions may need new fluid every 25,000 to 50,000 miles.

A transmission fluid icon.

Just like motor oil, there’s a dipstick for checking your transmission fluid. If your transmission fluid smells burnt or has visible flecks of metal or other debris, then it’s time to replace it. Transmission fluid starts out red but will get orange and then brown as it ages. If yours is dark brown or black, it’s time to replace it.

A quick note on transmission fluid: unlike motor oil, you have to turn your car on to check your transmission fluid. After starting your engine, keep the car in park and remove the dipstick.

Are Transmission Flushes Really Necessary?

Changing your fluid, also called a transmission flush, will eventually be necessary. Failing to do so can lead to straining and overheating.

Some automatic transmission fluid is marketed as a "lifetime" fluid. While this does mean that the fluid is hardy, saying you never need to replace it is an overstatement. You may never have to change it during the lifecycle of your car, but it isn’t indestructible. Be sure to check your vehicle's manual for the correct service schedule.

Differential Fluid

Your car’s differential contains moving gears and a clutch that distribute torque to the car’s wheels. Because of its moving parts, differentials need the same lubrication that an engine or transmission needs.

Differential fluid can degrade over time. Failing to replace it will cause the differential to become noisy, a sure sign that you need new fluid. If left unchecked, the gears will eventually seize up, which means that the wheels will lock together. This can cause damage to the differential gears or even be dangerous if it happens when you’re driving.

The other joints in your powertrain also need lubrication in order to work. Because of their construction, they can't hold in fluid, so a more viscous grease is used instead. Your differential fluid should be checked by your mechanic as part of a routine visit.

How Often Should You Change Your Differential Fluid?

While you don’t need to change it as often as your oil, differential fluid will need to be replaced at periodic intervals. Check your owner’s manual. It could be as often as every 50,000 miles or as seldom as every 150,000 miles.

Hydraulic Fluids

Another role that fluids play is transferring (and multiplying) power. These hydraulic fluids are non-compressible, which helps them transmit pressure. They’re what make power steering, brakes, and clutches work. If any of these fluids become contaminated or leak, they can seriously impact your car's ability to drive.

Do Hydraulic Fluids Need to Be Changed?

A brake fluid reservoir.

Hydraulic fluids do need to be periodically replaced. While these fluids don't get used up or break down like motor oil, they can still degrade over time. Some drivers think that a hydraulic fluid change is an unnecessary upcharge promoted by unscrupulous mechanics. It can certainly happen, but that isn't always the case.

There isn't any regular interval for changing these fluids, but it's a good idea to take a look at them from time to time. Check if you need to top off any of these fluids or potentially replace them entirely. The reservoirs for each are located in the engine bay.

Sometimes, specifically with brake and clutch fluid, air bubbles can be introduced into the system and ruin their performance. If you notice either pedal is unresponsive or spongy, then you should consider bleeding them.

Brake Fluid

Different cars use different brake fluids based on how performance-focused they are. Most brake fluids are DOT-3 or DOT-4. These are glycol-based and are great for most cars.

An ABS warning light can indicate a problem with your brake fluid.

Performance cars often need DOT-5, which is silicone-based. It’s more expensive, but it has a higher boiling point that makes it better in high heat conditions.

Brake fluid should be checked to ensure it’s not getting low in the reservoir.

How Do I Know If My Brake Fluid Is Bad?

A mushy, spongy-feeling brake pedal is a symptom that your brake fluid is low or needs to be replaced. You’re also likely to have worse braking performance. Old brake fluid will turn dark, like oil, and should be replaced at that point.

Naturally, this is an incredibly important area to monitor. It should be addressed immediately if there are any problems. Your ABS warning light may come on before any serious issues occur.

Power Steering Fluid

A power steering warning light often means an issue with the power steering fluid.

Though many vehicles have moved to electric steering, hydraulic steering is still used in many cars on the road. Power steering fluid is what powers the hydraulics of this system. You typically need to change it every 100,000 miles or as needed (check your owner’s manual). If your car has electric steering or no power steering, you don’t need to worry about this fluid.

How Do You Know If Power Steering Fluid Should Be Replaced?

If there’s an issue with the system, you’ll often feel a drastic increase in steering effort. It will feel like you’re trying to steer while the car is off. There could also be a rough feel when turning, or whining noises that occur when steering.

A symbol may appear on your gauge cluster as well. The dashboard warning light for a power steering issue is usually a steering wheel, often with an exclamation point beside it.

If any of these issues occur, it’s time to check your power steering fluid. The reservoir is under the hood, typically back towards the steering wheel. See if it’s low or if there’s a leak. It can be tough to “check” your power steering fluid regularly, and you likely won’t need to examine it until an issue crops up.

Clutch Fluid

Clutch fluid is simply brake fluid that’s kept in the clutch system’s reservoir. It’s used for hydraulic power in cars with manual transmissions. When the clutch pedal is pressed, a hydraulic piston disengages the clutch. Almost all modern cars have hydraulic clutch systems. Older cars have mechanical clutch systems but can be converted to hydraulic. If you have an automatic with no clutch (not a DCT) or a CVT, you don’t have to worry about clutch fluid.

How Do You Know If You Need Clutch Fluid?

Symptoms of bad or leaking clutch fluid include a clutch pedal that feels “off.” It could feel too heavy, too light, or like it won’t smoothly go through the full range of motion. You may also have rough gear shifts.

Coolants

Another function that car fluids perform in your vehicle is keeping everything nice and cool. Excess heat can lead to poor performance, or worse, damage to components. Fluids also keep the cabin cool in vehicles with air-conditioning.

Antifreeze/Coolant

The engine overheat warning light typically indicates a problem with your coolant or radiator. Stop driving immediately and pull over to a safe location.

Engines produce a lot of heat, and coolant is used to regulate it. Comprised of water and antifreeze, coolant circulates through the engine and the radiator, effectively transferring the heat to the air around the car. Without it, your engine would quickly overheat. This fluid will keep your engine cooler in the summer and warmer in freezing temperatures.

Virtually all modern engine coolants contain antifreeze. The active ingredient in antifreeze, glycol, prevents the water in your radiator and engine from freezing in cold conditions. It also prevents rust.

Coolant usually comes pre-diluted with a 50/50 mix of antifreeze and water. The combination is commonly referred to as antifreeze/coolant, engine coolant, radiator fluid, or radiator coolant. You’ll see “antifreeze/coolant” on most products though.

A coolant/antifreeze reservoir cap.

In standard weather conditions, antifreeze should be mixed 50/50 with water. In colder climates, it can be mixed as high as 70% antifreeze.

What Are the Signs of Low Coolant/Antifreeze?

Symptoms of a coolant issue include slower performance and steam coming from under the hood. Hopefully, you’ll get an overheat warning light on your dashboard well before that. Most cars also have gauges that monitor engine temperature.

Some vehicles use water-injection or water-cooling by spraying water directly into the engine’s cylinders. However, this is extremely rare for modern cars.

How Do I Know If I Need to Change My Coolant/Antifreeze?

Coolant quality can be difficult to measure. Check for rust around the cooling system, as this could indicate that the rust-preventing properties of the antifreeze are no longer working. If the coolant is dark brown, it’s definitely past its prime. Otherwise, a mechanic can check the acidity and cooling properties to assess the quality. Regardless, follow the instructions in your owner’s manual for the replacement interval.

AC Refrigerant and Freon

Another fluid that helps keep your car cool is the refrigerant in the AC system. Freon, the same fluid in refrigerators, is used to produce cold air. To fully understand this process, you’ll have to understand some basic thermodynamics. Changes in pressure cause changes in temperature. By compressing freon gas into a liquid, then compressing it back into a vapor, heat is transferred away from the AC system. This, in turn, creates cold air.

This is important to know because a broken AC system is one of the more frustrating parts of your car to break down. Sure, it can keep driving, but you might end up drenched in sweat when you get to your destination, depending on where you live.

How Do You Know If Your Car Needs Freon?

Freon periodically needs to be refreshed in your car. If your air conditioning has been functioning fine but now won’t dispense cold air, it probably needs freon. AC freon recharger kits are a simple and affordable way to fix this at home.

If the new freon doesn’t work, a larger issue, like a failed compressor or a leak, could be to blame. You should consult with your mechanic if the problem persists.

Not a critical warning, but it's important to have enough washer fluid to clean your windshield.

Windshield Wiper/Washer Fluid

Windshield wiper fluid is a simple liquid, but important for keeping your windshield clean. Depending on your climate, you can use water (with a touch of alcohol to kill bacteria). However, purpose-made windshield washing fluid is much better at cleaning your windows and keeping the fluid clean and unfrozen.

There are a variety of types to choose from. Some are designed to remove dead bugs, others are focused on de-icing. You can even choose a hydrophobic fluid that naturally repels rainwater.

Air in Tires

Yes, air is technically a fluid! While you might not think about it, be sure to check your tire pressure at least as often as you check your oil. Fill your tires to the manufacturer-recommended level, which can be found in your owner’s manual and on your door jamb.

Many gas stations offer free or very cheap air for your tires. You can also use an air compressor at home.

Though it can sometimes be triggered by cold air, a tire pressure light usually means one or more tires needs air.

Which Fluids Should You Check Often?

While all of the fluids we’ve talked about are important, some are more critical than others. They’ll get used up more quickly and can seriously ruin your drive if they run out. Here are the fluids you’ll want to check most often:

• Fuel
• Engine Oil
• Transmission Fluid
• Coolant
• Windshield Washer Fluid
• Air in Tires

In a modern vehicle, the car’s computer will monitor many fluids for you. Your mechanic should also be checking these fluids when you go in for routine or scheduled maintenance. However, alerts are not always reliable and don’t cover every fluid. Plus, some people enjoy doing their own maintenance, so it pays to know how to check your own fluids.

Keeping Your Car Fluids Topped Up

It might seem like a hassle to keep checking and refilling your fluids. It’s definitely not the most fun part of owning a car. But failing to monitor and replace your car’s fluids can lead to serious long-term damage. Doing some basic preventative maintenance will save you lots of money down the road. Not to mention you’ll be keeping the car in good shape for whoever gets it next.

Sources: Should I Change My Transmission Fluid, CarTalk | How to Change the 5 Essential Fluids In Your Car, Your Mechanic | How to Check the 6 Essential Fluids In Your Car, Popular Mechanics | How and Why You Should Check Your Transmission Fluid, Car and Driver | How to Check Fluid Levels, NAPA

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