These are the essential fluids that keep your car alive

And there is much, much more. Like the fluids in a car.

From the oil that keeps the engine properly lubricated (and even cooled in some designs), to the thin hydraulic fluid that goes to the brakes to handle the primary job of propelling the engine. And everything in between. Let’s avoid the flashing fluid jokes (even though it’s actually a thing) and instead quickly talk about all the common fluids found in cars today.

We won’t list them from most important to least important (spoiler alert: they’re all important!), but we won’t detail what DEF and windshield washer fluid are.

Oil is, for all intents and purposes, the lifeblood of the engine. Its job is to lubricate the complex system of bearings, seals and surfaces that keep it running smoothly, especially once it reaches operating temperature. The oil comes in various types (conventional, synthetic, semi-synthetic) and viscosities (5W-20, 10W-40, 0W-20, etc.); the best way to find out what is best for your engine is to read your oil. owner’s manual See this post for more information on viscosities.

Oil also operates an engine’s variable valve timing system, if equipped, and lubricates other components such as turbochargers.

As for its maintenance interval, every 3,000-5,000 miles is generally considered the standard. But again, check your owner’s manual. The oil breaks down and loses its effectiveness over time, and even varnishes and causes sludge to build up in the engine if not changed in time. Therefore, it is paramount to ensure that the things inside are always doing their job to the best of their ability.

At the opposite end of the continuous motion is controlling it, which is where the brake fluid comes in.

Brake fluid lives in a reservoir, is pressurized by the master cylinder, and reaches the brakes at each wheel via hard and soft lines.

Brake fluid service intervals vary, but there are a few simple ways to tell when it needs to be replaced. One is lower braking performance, usually coupled with longer/spongier brake pedal travel. Another is to open the tank and observe its color: the healthy liquid has a light straw color. If it’s dark brown, or darker, it’s time to change it. Another way is to use a simple tester, which analyzes how much moisture has accumulated in the fluid.

This slippery substance is hygroscopic, meaning it absorbs moisture, which over time reduces its effectiveness. Some important specifications to note about brake fluid is its wet and dry boiling point. Dry means when it’s cool and hasn’t had time to absorb moisture, while wet means when it has. The standard measurement for the wet boiling point is when the water contamination of the fluid is 3.7%.

There are DOT 3, DOT 4, or DOT 5 branch fluid, with the main difference being that the higher the number, the higher the dry and wet boiling points. Performance and racing applications typically see DOT 4 and 5 as both have higher dry and wet boiling points. The latter is actually considered hydrophobic (so doesn’t like moisture) because it’s silicon-based instead of glycol-based like the others. But it still breaks down and loses effectiveness over time.

When it comes to another vehicle movement, steering, many cars have what is called hydraulic power steering. It has largely given way to electrics, but that doesn’t mean it’s not still on the road. In fact, both my Audi S4 and BMW 128i have it. The root here is that hydraulic fluid, which is pressurized by a pump attached to the engine, helps you turn the wheel. Without it the steering is a real chore, ask me how I know after buying a Land Rover Discovery 1 with a disconnected power steering system.

However, it should not be confused with hydraulics brake fluid: These are not interchangeable. Power steering fluid has different ingredients and properties, and also has its own set of specifications. The best way to determine which is best for your car is, again, to read your owner’s manual, but they are usually pretty general. Such as green power steering fluid (such as Pentosin CHF11 and Liqui Moly central hydraulic system oil) is a common specification for European vehicles.

Again, service intervals vary, but are usually around every 50,000 miles. A quick way to check the condition of this fluid is to open the reservoir and remove a sample with something like a turkey baster, or simply read the dipstick: is it cloudy, cloudy, dark, brown, or anything else the color it has when was it new If so, it might be a good idea to change it.

Unless you’re driving something like an old Porsche or VW, your car is water-cooled, meaning there’s a mixture of water and coolant moving through a series of engine channels that the keep at operating temperature. This coolant is kept cool by being pumped through a series of hoses to the radiator at the front of the car. There’s more to it, like a thermostat that controls its flow, as well as its ability to provide heat to the cabin, but that’s the gist of it.

Coolant is mixed with deionized or distilled water, usually in a 50:50 ratio, although warmer climates may run more water to improve cooling. As long as you don’t do 80% water and then go downhill skiing for the weekend. This is because the main tasks of coolants are to prevent freezing, lubricate the cooling system and prevent oxidation.

Coolant also has a service interval: every 30,000 miles or three to five years. But again, confirm with your vehicle manufacturer what the ideal time frame is.

There are a wide variety of coolants, so always be sure to compare and contrast your owner’s manual with the bottle on the shelf. The most common way to tell them apart is color: most BMWs require blue coolant, while Volkswagen products take pink. But again, always confirm with official resources.

Automatic transmission fluid (ATF) has similar functions to engine oil: it cleans, lubricates, and acts as a way to hydraulically actuate components. Although, while motor oil does this in the, well, engine, ATF lives in the transmission, providing these services to the gears that receive the engine’s power to drive the car’s wheels.

Like oil, its operating temperature is crucial, as the higher its heat resistance, the smoother the shifts and overall operation. It will also help the transmission live a longer life.

Since ATF is a hydraulic fluid, it is also commonly used as a power steering fluid. However, not all cars are happy with this type of swap, so once again be sure to read your owner’s manual. Or, if your car has a community of enthusiasts, it could be a source of such knowledge as well.

Like other fluids we’ve looked at so far, there are many different types of ATF depending on the type of transmission they run through, so it’s important to choose one that works with yours. It is generally reddish in color and, like other fluids, darkens with age and wear, eventually turning brown. Change this fluid between 60,000 and 100,000 miles (check your manual for a more specific number). Be sure to check your car manufacturers’ resources for the exact type and capacity of your car.

This fluid fluid is for manual transmissions and serves a similar purpose to ATF, although it only provides lubrication. It is a particularly viscous and thick consistency, and is also famous for being a bit smelly.

Like motor oil, there are conventional, semi-synthetic and fully synthetic types. It also has a viscosity rating, such as 75W-90, 80W-90, etc. However, another specification is GL-1 through GL-5 from the American Petroleum Institute. The higher the number, the more focused and performance-oriented your application is. Mainly because of the amount of additives it has. Racing transmissions with spur gears that transfer a lot of torque may use GL-5, while conventional and everyday gearboxes usually see GL-4. But again, like all the other fluids we’ve reviewed, check your manufacturers recommendations for the best choice for your transmission.

Finally, there’s the differential oil, which is basically just gear oil. It is not uncommon for a manual transmission equipped car with its front engine and rear wheel drive, therefore separate units, to take a different gear oil for its differential than what is in its transmission. Especially a different GL rating. However, a transaxle, i.e. a unit comprising both the transmission and the differential, usually uses only one oil.

No one ever said that a car that you race and drive is a mere gimmick, and the fact that it could have five or more fluids running under its body certainly proves that.

But it’s important to at least know the basics of these fluids, especially what each one does and when to change it. We’ll continue to act like a broken record and say that checking your owner’s manual and manufacturer’s resources is always a good idea, and add that creating a quick cheat sheet to keep in your glove box is, too. Because knowledge, especially the quick reference kind, is always power.

Finally, even if there’s a sensor in your car ready to tell you when any of the above fluids are low, it’s still a good idea to check them regularly yourself (if you can). If any are low, it’s likely due to a leak that should be fixed sooner rather than later.