What Is a Differential?
The differential is a gearbox at the axle that allows the two wheels on an axle to rotate at different speeds — essential when cornering, because the outer wheel travels a longer distance than the inner wheel. Without a differential, your tyres would scrub and bind in every turn.
Most rear-wheel-drive vehicles have a rear differential. Front-wheel-drive vehicles integrate the differential into the transaxle. 4WD and AWD vehicles (very common in NZ — Toyota Hilux, Subaru Impreza, Suzuki Jimny) have two or three differentials: front, rear, and a centre (or transfer case).
What Is Differential Oil?
Differential oil (also called diff oil or gear oil) is a heavy-viscosity gear lubricant — typically 75W-90 GL-5 or 80W-90 GL-5 — that coats the ring gear, pinion gear, side gears, and bearings inside the differential housing. It:
- Prevents metal wear on hypoid gear teeth (which operate under extreme sliding pressure).
- Dissipates heat generated by gear mesh.
- Lubricates the bearings supporting the axle shafts.
Limited-slip differentials (LSDs) — fitted to performance vehicles, the Toyota Hilux, and some Subaru models — require a special LSD-compatible fluid or an LSD additive. Using standard GL-5 without LSD additive in an LSD causes the clutch packs to chatter, producing a "clunk-clunk-clunk" noise when cornering slowly.
Signs of Low or Degraded Differential Oil
- Whining or howling from the rear (or front) of the vehicle, especially under load or at certain speeds.
- Clunking on turn — particularly in an LSD that needs additive.
- Vibration through the drivetrain when accelerating.
- Visible oil leak from the diff housing or axle seals — look for dark oily residue around the rear axle flanges or the diff cover.
- Overheating of the diff housing (noticeable by touch after a drive).
Why NZ 4WD Owners Should Pay Particular Attention
NZ's terrain — river crossings, farm tracks, beach driving, mountain roads — puts differentials under far more stress than urban commuting. Water ingress during river crossings is a real risk; hot differential housings can suck in water past seals when submerged. Water-contaminated diff oil turns milky and loses its lubrication properties rapidly.
After any significant water crossing, it's worth having the diff oil inspected and changed if it shows any sign of contamination.
How Often to Change Differential Oil?
| Vehicle Type | Interval |
|---|---|
| Standard rear-wheel-drive | Every 50,000–100,000 km |
| 4WD front and rear differentials | Every 40,000–60,000 km |
| 4WD centre differential/transfer case | Every 40,000–60,000 km |
| After significant water crossing | Immediately after |
Many manufacturers specify a first change at 20,000–30,000 km on new vehicles to remove metal particles from the break-in period.
NZ Costs
| Service | Typical NZ Cost |
|---|---|
| Single differential oil change | $80–$160 |
| Front and rear differential service (4WD) | $180–$320 |
| Transfer case fluid change | $80–$150 |
| Axle seal replacement (if leaking) | $150–$350 per seal |
These costs are modest compared to a differential rebuild ($800–$2,500) or replacement ($1,500–$4,000+).
When to Book a Mechanic
- You hear whining or howling from the axle area.
- Clunking in an LSD on tight turns.
- You've done any off-road driving involving water crossings.
- Service is overdue — particularly if you've recently purchased a used import with unknown service history.