Using the correct brake fluid matters - the wrong type can damage the system. The good news: your car tells you exactly what it needs, and for most vehicles it is a simple answer. Here is how to know what brake fluid your car takes.
Check the reservoir cap and manual
The single easiest way: look at the brake fluid reservoir cap - it is almost always stamped with the required fluid, like "DOT 3" or "DOT 4." Your owner's manual lists it too. Always use what the vehicle specifies; do not guess or substitute a different type.
Most cars: DOT 3 or DOT 4
The vast majority of cars and trucks use glycol-based DOT 3 or DOT 4 - see DOT 3 vs DOT 4. DOT 4 has a higher boiling point and is common on newer vehicles and those with ABS or heavier braking demands. These two are generally compatible with each other, but always follow the spec.
The ones to be careful with
DOT 5.1 is glycol-based (mixes with 3/4), but DOT 5 is silicone and must NEVER be mixed with glycol fluids - see DOT 5 brake fluid and brake fluid compatibility. DOT 5 is only for specific applications like some classics and military vehicles; ordinary cars do not use it.
Buy fresh and sealed
Use only the specified type from a sealed container - brake fluid absorbs moisture once opened. When in doubt, match the cap. Keep fluid fresh with a periodic flush and pair it with quality pads and hardware.