Modern cars depend on half a dozen specialized fluids, and each one is engineered for a specific job and chemistry. Topping up with the wrong one is one of the most common and most expensive DIY mistakes: the wrong coolant can corrode an aluminum radiator, the wrong transmission fluid can ruin a CVT, and silicone brake fluid in the wrong system can damage seals. This guide covers what each fluid does, how to read the specifications, how to check levels, typical change intervals and which fluids are compatible with each other.

Engine oil

Engine oil lubricates, cools and cleans the engine. Two pieces of information on the bottle matter:

• Viscosity grade, such as 0W-20 or 5W-30. The first number (with W, for winter) describes cold-flow behavior; the second describes viscosity at operating temperature. Many newer engines specify thin oils such as 0W-20 or 0W-16, and some now call for 0W-12 or 0W-8.
• Performance specification. The American Petroleum Institute's current gasoline category is API SQ, and the matching ILSAC standard is GF-7; both were first licensed on March 31, 2025, according to MOTOR's report on the API announcement. GF-7A oils carry the API "Starburst" and can be used where older GF-6A or earlier oils were specified. GF-7B oils carry the API "Shield" and are meant for engines that specifically call for SAE 0W-16.

Some automakers require their own approvals on top of API and ILSAC, such as GM's dexos specifications or European manufacturer approvals. If your manual lists one, look for it on the label. Diesel engines use separate API "C" categories.

How to check engine oil

1. Park on level ground. Follow the manual on whether to check warm or cold; many say to shut the engine off and wait several minutes.
2. Pull, wipe and reinsert the dipstick. Pull it again and read the level between the marks.
3. Top up gradually. Add small amounts of the specified oil and recheck; overfilling can cause foaming and leaks.
4. No dipstick? Some vehicles use an electronic level check in the instrument cluster or infotainment menu.

Coolant (antifreeze)

Coolant carries heat away from the engine, protects against freezing and boiling, and, critically, prevents corrosion inside aluminum, iron, copper and solder components. That corrosion protection comes from an additive package, and the packages differ:

Do not choose by color. Manufacturers use different dyes, and two coolants of the same color can be chemically different. Mixing types generally reduces corrosion protection and service life, so if you are unsure what is in the system, drain and refill with the specified coolant. "Universal" coolants claim compatibility with many systems, but an OEM-spec coolant is the safest choice.

• Mix ratio: most systems use a 50/50 mix of concentrate and distilled water, or a premixed product. A 50/50 ethylene glycol mix typically protects to around −34°F (−37°C).
• Intervals: long-life coolants commonly go 100,000 miles or more for the first change; older formulas often called for every 2 to 3 years.

Automatic transmission fluid (ATF) and CVT fluid

ATF is a hydraulic fluid, a lubricant and a friction modifier all at once, which is why transmission makers specify fluids so precisely. Common examples include GM Dexron VI, Ford Mercon LV, Chrysler ATF+4, Toyota WS and Honda ATF DW-1. Continuously variable transmissions (CVTs) use dedicated CVT fluids, and dual-clutch transmissions (DCTs) use their own fluids too.

• Do not mix ATF and CVT fluid. A CVT relies on specific friction properties between its belt or chain and pulleys.
• "Multi-vehicle" fluids: they cover many conventional automatics, but check that the bottle explicitly lists your required specification.
• Checking: many modern transmissions have no dipstick and require a check plug procedure at a specific fluid temperature, often measured with a scan tool. Follow service information exactly.
• "Lifetime" fill: some manufacturers specify no routine change under normal use, but many recommend changes under severe service such as towing. See our normal vs. severe maintenance guide.

Brake fluid: DOT 3, DOT 4, DOT 5 and DOT 5.1

Brake fluid transmits pedal force to the calipers and must resist boiling under heavy braking. Federal standard FMVSS No. 116 sets minimum boiling points. "Dry" means fresh fluid; "wet" means fluid that has absorbed moisture in testing, which is closer to what is in an older car.

Key points:

• Glycol fluids absorb moisture. Over time, water lowers the boiling point and promotes internal corrosion. Many automakers recommend replacing brake fluid every two to three years; check your manual.
• Mixing glycol grades is safe but dilutes performance. Adding DOT 3 to a DOT 4 system leaves you with a fluid closer to DOT 3 performance. Use the grade on the reservoir cap or a higher glycol grade if your manual allows.
• DOT 5 silicone is different. FMVSS 116 requires it to be dyed purple. It does not absorb water and is used mainly in some classic and military vehicles. It is not interchangeable with glycol fluids and is generally not used in modern ABS-equipped cars.
• Paint damage: glycol brake fluid strips paint, so cover fenders and wipe spills with water immediately.
• Use sealed bottles. An opened bottle absorbs moisture from the air; buy small containers.

Replacing pads? Our brake pad and rotor guide covers checking fluid level afterward.

Other fluids to know

Power steering fluid

Many newer vehicles use electric power steering with no fluid at all. Hydraulic systems use either a specific power steering fluid or a specified ATF; some European systems use a mineral-based or synthetic hydraulic fluid that must not be mixed with ATF. Check the reservoir cap and manual.

Gear oil

Differentials and transfer cases commonly use API GL-5 gear oils such as 75W-90 or 75W-85, and limited-slip differentials may need a friction modifier additive. Some manual transmissions specify GL-4 or a dedicated manual transmission fluid, because certain GL-5 additives can be harsh on older synchronizer materials.

Windshield washer fluid

Use a winter-rated fluid in freezing climates; plain water can freeze in the reservoir and lines.

Identify a leak by color

Fluid check checklist

Use our maintenance interval planner to track when each fluid change is due.