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There's a lot that goes into starting up your car and safely driving it down the road. The ignition system is a primary part of the equation, as is feeding the engine fuel and air at the proper ratio to ensure it'll perform as efficiently as it can. This means there's a complex system of computers and sensors thrown in as well.
And there's so, so much more. Like a car's fluids.
From the oil keeping the engine properly lubricated (and even cooled in some designs), to the thin hydraulic fluid coursing to the brakes to reign in the engine's primary job of propulsion. And everything in between. Let's avoid the blinker fluid jokes (though that's actually kind of a thing), and instead, quickly discuss each and every common fluid found in cars today.
We won't outline them from most important to least important (spoiler alert: They're all important!) but will skip detailing what DEF and windshield washing fluid are.
Oil is, for all intents and purposes, the lifeblood of the engine. Its duty is to lubricate the complex system of bearings, seals, and surfaces that keep it running smoothly, especially once it's up to operating temperature. Oil comes in various types (conventional, synthetic, semi-synthetic) and viscosities (5W-20, 10W-40, 0W-20, and so on)—the best way to figure out which is best for your engine is to read your owner's manual. Check out this post for more on viscosities.
Oil also actuates an engine's variable valve timing system, if it's equipped, and lubricates other components like turbochargers.
As far as its maintenance interval, every 3,000-5,000 miles is generally considered the standard. But once again, check your owner's manual. Oil breaks down and loses its effectiveness over time, and will even varnish and cause sludge buildup in the engine if not changed in time. So it's paramount to ensure the stuff that's in there is always doing its job to the best of its ability.
On the opposite end of continued motion is controlling it, which is where brake fluid comes in.
Brake fluid lives in a reservoir, is pressurized by the master cylinder, and reaches the brakes on each wheel via hard and soft lines.
Brake fluid service intervals vary, but there are some simple ways to tell when it's in need of replacement. One is poorer braking performance, usually joined by a longer/spongier brake pedal travel. Another is to open the reservoir and observe its color: healthy fluid 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 built up in the fluid.
This slippery substance is hygroscopic, meaning it sucks up moisture, which over time reduces its effectiveness. Some important specs to keep in mind about brake fluid is its wet and dry boiling point. Dry means when it's fresh and hasn't had time to absorb moisture, whereas wet means when it has. The standard measure for wet boiling point is when the fluid's water contamination is at 3.7%.
There's DOT 3, DOT 4, or DOT 5 brank fluid, with the main difference being the higher the number, the higher the dry and wet boiling points. Typically, performance and racing applications see DOT 4 and 5 as they both have higher dry and wet boiling points. The latter is actually considered hydrophobic (so, it doesn't like moisture) due to being silicon-based instead of glycol like the others. But it still breaks down and loses effectiveness over time.
Power Steering Fluid
When it comes to another vehicular motion, steering, many cars have what's called hydraulic power steering. It's largely given way to electricity, 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 via a pump attached to the engine—helps you turn the wheel. Without it, steering is a real chore, ask me how I know after I bought a Land Rover Discovery 1 with a disconnected power steering system.
Though, it's not to be confused with hydraulic brake fluid—these aren't 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, yet again, to read your owner's manual, but they're usually pretty general. Such as green power steering fluid (like Pentosin CHF11s and Liqui Moly Central Hydraulic System Oil) being a common spec for European vehicles.
Once again, service intervals vary, but it's usually somewhere around every 50,000 miles. A quick way to check the condition of this fluid is to open the reservoir and pull out a sample with something like a turkey baster, or just read the dipstick: Is it cloudly, murky, dark, brown, or anything other than the color it was when new? If so, it might be a good idea to change it.
Coolant, Otherwise Known As Anti-Freeze
Unless you're tooling around in something like an old Porsche or VW, your car is water-cooled, meaning there's a mixture of water and coolant moving around through a series of channels in the engine keeping it at operating temperature. This coolant stays cool by pumping through a series of hoses to the radiator at the front of the car. There's more to it, such as a thermostat that controls its flow, as well as its ability to provide heat to the cabin, but this is the general gist of it.
Coolant mixes with deionized or distilled water, usually at a ratio of 50:50, though warmer climates can run more water for improved cooling. So long as you don't run 80 percent water and then head up to some high-elevation skiing for the weekend. That's because coolants' main jobs are to prevent freezing, lubricate the cooling system, and prevent rust.
Coolant also has a service interval: Every 30,000 miles or three-to-five years. But again, confirm with your vehicle's manufacturer what the most ideal time frame is.
There's a wide variety of coolants out there, 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, whereas Volkswagen products take pink. But again, always confirm with official resources.
Automatic Transmission Fluid (ATF)
Automatic transmission fluid (ATF) plays similar roles as engine oil: It cleans, lubricates, and acts as a way to hydraulically actuate components. Though, whereas engine oil does so in the, well, engine, ATF lives in the transmission, providing these services to the gears that receive power from the engine to drive the car's wheels.
Also like oil, its operating temperature is crucial, as the higher its resistance to heat, the smoother shifts and overall operation will be. Plus it'll help the transmission live a longer life.
Because ATF is a hydraulic fluid, its also commonly used as a power steering fluid. However, not all cars are happy with this kind of swapping around, so be sure to, yet again, read your owner's manual. Or, if your car has an enthusiast community, that could also be a source for this kind of knowledge.
Like other fluids we've gone over so far, there are many different types of ATF depending on the type of transmission they're flowing through, so it's important to pick one that works with yours. It generally has a reddish color, and like other fluids it gets darker the older and more worn out it gets, eventually turning brown. Change this fluid anywhere between 60,000 to 100,000 miles (check the manual for a more specific number). Be sure to consult automakers' resources for your exact car's type and capacity.
This humble fluid is for manual transmissions and serves a similar purpose as ATF, though only provides lubrication. It's a particularly viscous, thick consistency, and is infamous for being a bit stinky, too.
Like engine oil, there are conventional, semi-synthetic, and full-synthetic kinds. It has a viscosity rating, too, such as 75W-90, 80W-90, and so on. However, another specification is the American Petroleum Institute's GL-1 through GL-5. The higher the number, the more focused and performance-oriented its application is. Mainly due to the number of additives it possesses. Racing transmissions with straight-cut gears that transfer a lot of torque might use GL-5, whereas conventional, everyday gearboxes tend to see GL-4. But again, like every other fluid we've gone over, consult your manufacturers' recommendations for the best choice for your transmission.
Finally, there's differential oil, which is basically just gear oil. It's not uncommon for a manual-transmission-equipped car with its engine in front and rear-wheel drive—so, separated units—to take a different gear oil for its differential than what's in its transmission. Especially a different GL rating. However, a transaxle—meaning, a unit that comprises both the transmission and differential—usually uses just one oil.
Lots of Fluids to Think About
Nobody ever said that a running and driving car is a simple contraption, and the fact that it could have five-or-more fluids coursing around under its bodywork certainly proves this.
But it's important to at least know the basics of these fluids, particularly what each one does and when to change it. We'll continue acting like a broken record and say that consulting your owner's manual and manufacturer's resources is always a good idea, and add that whipping up a quick cheat sheet of it all to keep in the glovebox is, too. Because knowledge, especially the quick-to-reference kind, is always power.