Everyone remembers the engine oil. It's the other fluids that quietly decide whether a car survives — and they're exactly the ones owners skip and buyers never check. Here's what each one does, the mistakes that wreck engines and gearboxes, and what to look at before you buy.
Coolant (antifreeze) keeps the engine at the right temperature and, just as importantly, stops the inside of the cooling system rusting. In SA's heat that second job matters as much as the first. Two mistakes are common and both are expensive.
The first is topping up with plain water. Water alone has no corrosion protection and dilutes what's there, so the system slowly rusts from the inside. The second is mixing coolant types. Coolants come in different chemistries — the older silicate-based (often green) and the newer long-life OAT/HOAT types (often pink, red or blue). Mixing incompatible types can cause the coolant to gel or turn to sludge, blocking the system. The colour is a rough guide, not a guarantee — match the type your car specifies, don't just match the colour.
"It's low, I'll just add water." A small top-up of water in an emergency is fine to get you home, but routinely running water instead of the correct coolant mix invites corrosion and overheating. Top up with the right pre-mixed coolant, and have the system flushed and refilled on the manufacturer's schedule.
This is the one almost nobody changes, because nothing feels wrong. Brake fluid is hygroscopic — it absorbs moisture from the air over time. As it takes on water, its boiling point drops and it starts corroding the brake system from the inside. Under hard or repeated braking, that absorbed water can boil and the pedal goes soft — brake fade, exactly when you need the brakes most.
Because this is driven by time and moisture rather than how far you drive, manufacturers typically call for a brake fluid change roughly every two years regardless of mileage. A car that's done low kays can still have brake fluid well past its useful life. Check your manual for the exact interval and the required DOT rating.
Automatic transmission fluid (ATF) lubricates, cools and actually operates the gearbox. Many modern autos are marketed as "sealed for life" or "fill for life" — and a lot of owners take that to mean the fluid never needs attention.
In practice, "life" often means the manufacturer's expected service life, not yours, and plenty of automatics — including modern CVT and dual-clutch boxes — run better and last longer with periodic fluid changes using the exact specified fluid. Neglected ATF shows up as harsh or hesitant shifts and, eventually, a gearbox failure that costs far more than a fluid service ever would. The catch is that these boxes are fussy about the precise fluid, so this is one to leave to someone who knows the specification — not a generic top-up.
"It's sealed for life, so the gearbox needs nothing." Treat "sealed for life" with caution. Check what the manual actually says about transmission servicing — ignoring it can turn a modest fluid change into a major gearbox bill.
If you're buying a bakkie or 4x4, there are extra oils most people never think about: the differential oil and, on four-wheel-drive vehicles, the transfer case oil. These quietly wear if never changed, especially on a vehicle that's towed, carried loads or seen any off-road or water crossings. Ask whether they've ever been serviced — on a hard-working vehicle, "never" is a small red flag.
| Fluid | What it does | Warning sign |
|---|---|---|
| Coolant | Temperature control + corrosion protection | Rusty/brown colour, low level, oily film (possible head gasket) |
| Brake fluid | Transfers braking force; resists heat | Dark/black fluid, soft or sinking pedal |
| Transmission fluid | Lubricates and operates the gearbox | Burnt smell, very dark fluid, harsh or hesitant shifts |
| Diff / transfer case oil | Lubricates drivetrain (bakkies/4x4s) | Whining or graunching from underneath, never serviced |
You don't need tools — most of this is a look under the bonnet and a careful test drive:
The full fluid inspection — reservoirs, leaks and the test-drive feel — is covered step by step in our Engine Bay and Test Drive inspection sections.
Never run plain water as coolant or mix incompatible types; treat brake fluid as something that expires every couple of years no matter the mileage; and don't trust "sealed for life" — check what the manual really says about the gearbox. When buying, glance at the reservoirs and feel the shifts and the brake pedal. These are small checks guarding against some of the biggest bills.
Every check that matters before you hand over money — printable, and built for SA buyers.